Studies in the Sierra. No. V. - Post-Glacial Denudation.

• Main Author: Muir, John
• Format: Text
• Language: English
• Published: Scholarly Commons 1874
• Subjects: John Muir; Bibliography; Kimes; William F. Kimes; Maymie B. Kimes; pamphlets; journal articles; speeches; writing; naturalist; annotation; Cairn; Eyrie; Jagged; Lone; Rotten; Terra Firma; The Three Brothers; Three Brothers; Ice Sheet
• Online Access: https://scholarlycommons.pacific.edu/jmb/85; https://scholarlycommons.pacific.edu/cgi/viewcontent.cgi?article=1084&context=jmb

I THE Overland Monthly DEVOTED TO THE DEVELOPMENT OF THE COUNTRY. Vol. 13.—NOVEMBER, 1874.—No. 5. v STUDIES IN THE SIERRA. NO. V. —POST-GLACIAL DENUDATION. WHEN Nature'lifted the ice-slieet from the mountains/she may well be saidy not to have turned a new leaf, but to have made a new one of the old. Throughout the unnumbered seasons of the glacial epoch the range lay crushed and sunless. In the stupendous denudation to which it was then subjected, all its pre-glacial features disappeared '"plants, animals, and landscapes were wiped from its flanks like drawings from a blackboard, and the vast page left smooth and clean, to be repictured with young life and the varied and beautiful inscriptions of water, snow, and the atmosphere. The variabilityT) hardness, structure, and mineralogical composition of the rocks forming the present surface of the range has given rise to irregularities in the amount of post-glacial denudation effected in different portions, and these irregularities have been greatly multiplied and augmented by differences in the kind and intensity of the denuding forces, and in the length of time that different portions of the range have been exposed to their action. The summits have received more snow, the foothills more rain, while the middle region has been variably acted upon by both of these agents. Again, different portions are denuded in a greater or less degree according to their relations to level. The bottoms of trunk valleys are swept by powerful rivers, the branches by creeks and rills, while the-intervening plateaus and ridges are acted upon only by thin, feeble currents, perfectly silent and nearly invisible. Id-like manner some portions of the range are subjected every winter to the scouring action of avalanches, while others are entirely beyond the range of such action. But the most influential of the general causes that have conspired to produce irregularity in the quantity of post-glacial denudation is the difference in the length of time during which different portions of the range have been subject- Entered according to Act of Congress, in the year 1874, by John H. Carmany, in the. Office of the Librarian of Congress, at Washington. Vol. 13. — 26. KaWi- : 394 STUDIES IN THE SIERRA. [Nov. ed to denuding agents. The ice-sheet melted from the base of the range tens of thousands of years ere it melted from the upper regions. We find, accordingly, that the foot-hills are heavily blurred, while "the alps/and a considerable portion of the middle region remain fresh and shining as if they had never suffered from the touch of a single storm. Perhaps the least known among the more outspoken agents of mountain degradation are those currents of eroding rock called avalanches. Those of the Sierra are of all sizes, from a few sand- grains or crystals worked loose by the weather and launched to the bottoms of cliffs, to those immense earthquake avalanches that thunder headlong down amid fire and smoke with a violence that shakes entire mountains. Many avalanche-producing causes, as moisture, temperature, winds, and earthquakes, are exceedingly variable in the scope and intensity of their action. During the dry, equable summers of the middle region, atmospheric disintegration goes silently on, and many a huge mass is made ready to be advantageously acted upon by the first winds and rains of winter. Inclined surfaces are then moistened and made slippery, decomposed joints washed out, frost-wedges driven in, and the grand avalanche storm begins. But though these stone- storms occur only in winter, the attentive mountaineer may have the pleasure of witnessing small avalanches in every month of the year. The first warnings of the bounding free of a simple avalanche is usually a dull muffled rumble, succeeded by a ponderous crunching sound; then perhaps a single huge block weighing several tons may be seen wallowing heavily down the face of a cliff, followed ' by a train of smaller stones, which are gradually left behind on account of the greater relative resistance they encounter as compared with their weight. The , eye may therefore follow the large block undisturbed, noting its awkward, lumbering gestures as it gropes its way through the air in its first wild journey, and how it is made to revolve like a star upon its axis while it pursues the grand smooth curves of general descent. Where it strikes a projecting boss it gives forth an intense gasping sound, which, coming through the darkness of a storm- night, is indescribably impressive; arid when at length it plunges into the valley, the ground vibrates as if shaken by an. earthquake. '"" On the 12th of March, 1873, I witnessed a magnificent avalanche from the face of the second of the Three Brothers; in Yosemite Valley. A massive stream of blocks bounded from ledge to ledge and plunged into the talus with a display of energy inexpressibly wild and exciting. Fine gray foam- dust boiled and swirled along its path, and gradually rose iar, above the top of the cliff, appearing as a dusky cloud on the cairn still azure.-"Unmistakable traces of similar avalanches are visible here, probably caused by the decomposition of the feldspathic veins with which the granite is interlaced. Earthquakes, though not of very, frequent occurrence in the Sierra, are powerful causes of avalanches. Many a lofty tower and impending brow were left in delicate poise by the glaciers, and stood firm through the storms of the first post-glacial seasons. Torrents swept their bases, and winds and snows slipped glancingly down their polished sides, without much greater erosive effect than the passage of cloud-shadows. But at length, the new-born mountains were shaken by an earthquake - storm, and d .thousand-'weak forms staggered and fell in-one simultaneous crash. The records of this first post-glacial earthquake present themselves in every cafion and around the bases of every summit alp that I have visited; and it is a fact of great geological interest that ,„,-. v 1874-] STUDIES IN THE SIERRA. 395 to it alone more than nine - tenths of all the cliff taluses,are clue. The largest of these earthquake taluses measure from 500 to 1,000 feet in height, and are timbered with spruce, pine, and live-oak over their entire surfaces, showing that they have not been disturbed since their formation,.either by denudation, or accessions of fresh material. The earthquake which destroyed the village of Lone Pine, in March, 1872, shook the Sierra with considerable violence, giving rise to many new taluses, the formation of one of which I was so fortunate as to witness. The denuding action of avalanches is not unlike that of water-torrents. They are to-be. frequently seen descending the summit peaks, flowing in regular channels, the surfaces of which they erode by striking off large chips and blocks, as well as by wearing off sand and dust. A considerable amount of grinding also goes on in the body of the avalanche itself, reducing the size of the masses, and preparing them for the action of other agents. Some avalanches hurl their detritus directly into the beds of streams, thus bringing it under the influence of running water, by which a portion of it is carried into the ocean. The range of rock avalanches, however produced, is restricted within comparatively narrow bounds. The shattered alps are constant fountains, but the more powerful mountain-shaking avalanches are confined to the edges of deep Yosemitic-cafions in a zone twelve or fifteen miles wide, and gradually merge into land-slips along their lower limits. Large rock avalanches pour freely through the air from a height of hundreds or thousands of feet, and on striking the bottom of the valley are"dashed into a kind of coarse stone foam. Or, they make the descent in several leaps, or rumble over jagged inclines in the form of cascades. But in any case they constitute currents of loose-flowing fragments. Land-slips, on the contrary, slip in one mass, and, unless sheer cliffs lie in their paths, may come to rest right- side up and undivided. There is also a marked difference in their geographical distribution, land-slips being restricted to deeply eroded banks and hill-sides of the lower half of the range, beginning just where rock avalanches cease. Again, the material of land-slips is chiefly fine soil and decomposing bowlders, i*7 while that of rock avalanches is mostly of taw-stone. Y Fig. 1. Let Fig. 1 represent a section across a valley in which moraine matter, A, is deposited upon the inclined bed-rock, B B B. Now, strong young moraine matter deposited in this way, in a kind of rude masonry, always rests, or is capable of resting at a much steeper angle than the same matter after it has grown old and rotten. If a poultice of acid mud be applied to a strong bowlder, it will not be much affected in an hour or clay, but if kept on for a few thousands or tens of thousands of years, it will at length soften and crumble. Now, Nature thus patiently poultices the bowlders of the moraine banks under consideration. For'many years subsequent to the close of the ice period very little acid for this purpose was available, but as vegetation increased and decayed, acids became more plentiful, and bowlder decomposition went on at an accelerated rate, until a degree of weakness . dM 396 was induced that caused the sheerest portions of deposits, as A B D (Fig. 1), to giye way, perhaps when jarred by an earthquake, or when burdened with snow or rain, or partially undermined by the action of a stream. It appears, therefore, that the main cause of the first post-glacial land-slips is old age. They undoubtedly made their first appearance in moraine banks at the foot of the range, and gradually extended upward to where we now find them, at a rate of progress measured by that of the recession of the ice-sheet, and by the durability of moraines and the effectiveness of the corroding forces brought into action upon them. In those portions of the Sierra where the mo- rainal deposits are tolerably uniform in kind and exposure, the upper limits of the land-slip line is seen to stretch along the range with as great constancy of altitude as that of any snow-line. The above-described species of landslip is followed up the range by another of greater size, just as the different forest trees follow one another in compliance with conditions of soil and climate. After the sheer end of the deposit (A B D, Fig. 1) has slipped, the whole mass may finally slip on the bed-rock by the further decomposition, not only of the deposit, but of the bed-rock on which it rests. Bed - rocks are usually more or less uneven. Now it is plain that when the inequalities B B B crumble by erosion, that the mass of the deposit will not be so well supported; moreover, the weight of the mass will continue to increase as its material is more thoroughly pulverized, because a greater quantity of moisture will be required to saturate it. Thus it appears that-as the support of moraine deposits diminishes, the necessity for greater support increases, until the-concurrence of a cluster of minor causes finally brings on a slip. Slips of this species are often of great STUDIES IN THE SIERRA. extent, the surface of the moving mat .- >v% [Nov. . ■ lovinf mat- , '■ If -tw comprising several acres overgrown ■ with trees;' sowfe doming to rest with all their load of vegetation uninjured,' leaving only a yawning rent to mark . m their occurrence.-' Land-slides occur ;, ' ■ft more frequently on the north than on the south sides of ridges, because of the ' greater abundance of weight-producing and decomposing moisture. One of the commonest effects of land-slips is the clamming of streams, giving rise to large accumulations of water, which speedily burst the dams and deluge the valleys beneath, sweeping all the finer detritus before them. . - - .* at- ' The quantity of denudation accom-,-,, -. . plished by the Sierra land-slips of both LJJ. species is very small. Like rock-falls, they erode the surface they slip upon in ] a mechanical way, and also bring clown ''•* material to lower levels where it may be more advantageously exposed to the denuding action of other agents, and open scars whereby rain-torrents are enabled to erode gullies; but the sum of the areas thus affected bears an exceedingly small proportion to the whole surface of the range. The part which snow avalanches play in the degradation of mountains is simpler than that of free-falling or cascading rocks, or either species of land-slip; these snow avalanches being external and distinct agents. Their range, however, is as restricted as that of either of the others, and like them they only carry their detritus a short distance and leave it in heaps at the foot of cliffs and steep inclines. There are three well-marked and distinct species of snow avalanche in the upper half of the Sierra, differing widely in structure, geographical distribution, and in the extent and importance of the geological changes they effect. The simplest and commonest species is formed of young mealy snow, and occurs during and a short time after every heavy snow-fall wherever the mountain -I874-] STUDIES IN THE SIERRA. 397 ■ •■ ,v / ■ 9 ' slopes are inclined at suitable angles. This species is of .very- frequent occurrence throughout all the steep-flanked alps, where it reaches perfection, and is also common throughout the greater portion of the middle region. The/kva- lanches of-tlie-Sierra.lps are the feed- V ers of the glaciers, pouring down their dry mealy snow into the womb-amphitheatres, where it is changed to ndve' and ice. Unless disturbed by storm-winds, they cascade down the jagged heights in regular channels, and glide gracefully out over the glacier slopes in beautiful parallel curves; which action gives rise in summer to a mostnteresting and comprehensive systercl of snow-sculpture. The detritus discharged upon the surface of the glaciers forms a kind of stone- drift which is floated into moraines like the straws and chips of rivers. Few of the defrauded toilers of the plain know the magnificent exhilaration of the boom and rush and outbound- ing energy of a great snow avalanche. While the storms that breed them are in progress, the thronging flakes darken the air at noonday. Their muffled voices reverberate through the gloomy canons, but we try in vain to catch a glimpse of their noble forms until rifts appear in the . —azur.e.-sky, and the storm ceases. Then in cliff- walled valleys like Yosemite we may witness the'descentof half a dozen or more,within a few hours. -A The denuding power of this species of avalanche is not great, because the looseness of the masses allows them to roll and slip upon themselves. Some portions of their channels, however, pre- : sent a rough, scoured appearance, caused by rocky detritus borne forward in the under portion of the current. The avalanche is, of course, collected in a heap at the foot of the cliff, and on melting leaves the detritus to accumulate from year to year. These taluses present striking contrasts to those of rock avalanches caused by the first great post glacial earthquake. The latter are gray in color, with a covering of slow-growing lichens, and support extensive groves of pine, spruce, and live-oak; while the former, receiving additions from year to year, are kept in a raw formative state, neither trees nor lichens being allowed time to grow, and it is a fact of great geological significance that no one of the Yosemite snow avalanches, although they have undoubtedly flowed in their present channels since the close of the glacial period, has yet accumulated so much debris as some of the larger earthquake avalanches accumulated-in a few seconds. The next species of avalanche in natural order is the annual one, composed of heavy crystalline snows which have been subjected to numerous alternations of frost and thaw. _ 398 STUDIES IN THE SIERRA: of their courses with a hard snout kept close down on the surface of the rock, and because the middle of the snout is stronger, the detritus heaps are curved after the manner of terminal moraines. These detritus heaps also show an irregularly corrugated and concentric structure. An examination of the avalanche pathways shows conclusively that the annual accretions of detritus, scraped from their surfaces, are wholly insufficient to account for the several large concentric deposits. But when, after the detritus of many years has been accumulated by avalanches of ordinary magnitude, a combination of causes, such as rain, temperature, and abundant snow-fall gives rise to an avalanche of ex-'' traordinary size, its superior momentum will carry it beyond the limits attained by its predecessors, and furnish it with an opportunity of sweeping-forward theljjp several deposits into a single eoncentrie >. mass. A succession of these irregular- '• ,' ities will obviously produce results corresponding in every particular with the observed phenomena. What we may call century avalanches, as distinguished from annual, are conceived and nourished on cool mountain sides 10,000 or 12,000 feet in height, where the snow falling from winter to winter will not slip, and where the exposure and temperature is such that it will not always melt off in summer. Snow .accumulated under these conditions may linger without seeming to greatly changeinjnany years, until some slowly organized group of causes, such as temperature, abundance of snow, condition of snow, or the mere occurrence of an earthquake, launches the grand mass. In swooping down their mountain flanks they usually strip off the forest that liSs in their way, as well as the soil on which it-is. growing. Some of these forest pathways are 200 yards wide, and extend from the upper limit of the tree-line to the bottom of the valleys. They are all well "blitzed" on both sides by descending trunks, many of which carry sharp stones clutched in their up-torn roots. The height of these "blazes" and-gashes measures the depth of the avalanche at the sides, while in rare instances some noble silver-fir is found standing out in the channel, the only tree sufficiently strong to withstand the mighty onset; the scars upon which, or its broken branches, being- the record).of the depth of the cur- rent at that. plaqjK The ages of the trees show that some of these colossal avalanches occur only once in a. century, or more-seldom. These avalanches are by far the most pogHglul of the three species, although frorrrnhe rarity of their occurrence and the narrowness of the zone in which they find climatic conditions suited to their, development, the sum of the denudation accomplished by them is less than that of either of the others. We baveglsen that water in the condition of raf: dew, vapor, and melting snow, combined with air, acts with more or less efficiency in corroding the whole mountain surface, thus preparing it for the more obviously mechanical action of winds, rivers, and avalanches. Running water is usually regarded as the most influential of all denuding agents. Those regions of the 'globe first laid bare by the melting of the ice-sheet present no unchan§ed glaciated surfaces, from which, measuring down, we may estimate the amount of post-glacial denudation. The streams of these old eroded countries are said by the poets to "go on forever" and the conceptions of some geologists concerning them are scarcely less vague. Beginning at the foot.of the Sierra glaciei'Sjand following the torrents that rush out from beneath them down the valleys, we find that the rocks over which they flow are weathered gradually, the mpre the farther we descend; t Ml 1874.] STUDIES IN THE SIERRA. 399 showing that the streams in coming into existence grew like trees from the foot of the range upward, gradually ramifying higher and wider as the ice-sheet was withdrawn — some of the topmost branchlets being still in the, process of formation. Rivers are usually regarded as irregular branching strips of running water, shaped somewhat like a tree stripped of its leaves. As far as more striking features and effects are concerned, the comparison is a good one; for in tracing .Sierra rivers to their fountains we observe that as their branches divide and redivide, they speedily become silent and inconspicuous, and apparently chan- nelless: yet it is a mistake to suppose that streams really terminate where they become too small to sing out audibly, or erode distinct channels. When we stoop down and closely examine any portion of a mountain surface during the progress of a rain-storm, we perceive minute water-twigs that continue to bifurcate until like the nettedjg?eins of leaves the innumerable currentle.ts disappear in A broad universal thallus. J It would appear, therefore, that Sierra rivers more nearly resemble certain gigantic alga with naked stalks, and branches webbed into a flat thallus. The long unbranch- ed stalks run through the dry foot-hills; the webbed branches frequently overspread the whole surface of the snowy and rainy alpine and middle regions, as well as every moraine, bog, and neve bank. The gently gliding rain-thallus fills up small pits as lakelets and carries away minute specks of dust and mica. Larger sand-grains are overflowed without being moved unless the surface be steeply inclined, while the rough grains of quartz, hornblende, and feldspar, into which granite crumbles, form obstacles around which it passes in curves. Where the currentlets concentrate into small rills, these larger chips and crystals are rolled over and over, or swept forward partly suspended, just as dust and sand- grains are by the wind. The transporting power of steeply inclined torrents is far greater than is commonly supposed. Stones weighing several tons are,swept down steep canon gorges and spread in rugged deltas at their mouths, as if they had been floated and stranded like blocks of wood. The denudation of gorges by the friction of the bowlders thus urged gratingly along their channels is often quite marked. Strong torrents also denude their channels by the removal of blocks made separable from the solid bed-rock by the development of cleavage planes. Instructive examples of this species of denudation may be studied in the gorges between the upper and lower Yosemite falls and the Tenaya Canon, four miles above Mirror Lake. This is the most rapid mode of torrent denudation I have yet observed, but its range is very narrowly restricted, and its general denuding effects inappreciable. Water-streams also denude mountains by dissolving them and carrying them away in solution, but the infinite slowness of this action is clearly exemplified by the fact, that in the upper portion of the middle region granite ice-planed pavements have been flowed upon incessantly since they were laid bare on the breaking up of the glacial winter without being either decomposed, dissolved, or mechanically eroded to the depth of the one - hundredth part of an inch. Wind-blown dust, mica flakes, sand, and crumbling chips are being incessantly moved to lower levels wherever wind or water flows. But even in the largest mountain rivers the movement of large bo\lclers is comparatively a rare occurrence. When one lies down on a river-bank opposite a bowlder- spread incline and listens patiently for a day or two, a dull thumping sound may r 400 STUDIES IN THE SIERRA. [Nov. X occasionally be heard from the shifting of a bowlder, but in ordinary times few streams do much boiylder work; all the more easily moved blocks having been adjusted and re-adjusted during freshets, when the current was many times more powerful. All the channels of Sierra streams are subjected to the test action of at least one freshet per season, on the melting of the winter snow, when all weakly constructed dams and drift- heaps are broken up and re-formed. It is a fact of great geological interest, that only that portion of the general detritus of post-glacial denudation—that is, in the form of mud, sand, fine gravel, and matter held in solution—has ever at any time, been carried entirely out of the ra'nge into the plains or ocean. In the canon of the Tuolumne River, we find that the chain of lake basins which stretch along the bottom from the base of Mount Lyell to the Hetch-Hetchy Valley, are filled with detritus, through the midst of which the river flows; but instead of (the washed bowlders, which form a large portion of this detritusfhe- ing constantly pushed forward from basin to basin, Jj-rey lie still for centuries at a time, as is strikingly demonstrated by an undisturbed growth of immense sugar-pines and firs inhabiting the river banks. But the presence of these trees upon water-washed bowlders only shows that no displacement has been effected among them for a few centuries. They still must have been swept forward, and outspread in some grand flood prior tattle planting of these trees. But even this grand old flood-wliose magnificent traces occur everywhere on both flanks of the range, did not remove a single bowlder from the higher to the lower Sierra in that section of the range drained by the Tuolumne and Merced, much less into the ocean, because the lower portion of the Hetch-Hetchy B'asin, situated about half-way clown the western flank, is still in process of filling up, and as yet contains only sand and mud to as great a depth as observation can reach in river sections. The river flows slowly through this alluvial deposit and out of the basin over a lip of solid bedrock, showing that not a. single high Sierra bowlder ever passed it since the close of the glacial period; and the same evidence is still more strikingly exhibited in similarly situated basins in the Merced Valley. Frost plays a very inferior part in Sierra degradation. The lower half of the range is almost entirely exempt from its disruptive effects, while the upper half is warmly snow-mantled throughout the winter months. At high elevations of from ten to twelve thousand feet, sharp frosts occur in the months of October and November, before much snow has fallen; and where shallow water currents flow over rocks traversed by open divisional joints, the freezing that ensues forces the blocks apart and produces an/ exceedingly-ruinous appearance, without effecting much absolute displacement. The blocks thus loosened are, of course, liable to be moved by flood currents. This action, however, is so lim- ite'd in range, that the general average result is inappreciable. Atmospheric weathering has, after all, done more to blur and degrade the glacial features of the Sierra than all other agents combined, because of the universality of its scope. No mountain escapes submergence in the atmosphere, or fails to feel its decomposing and mechanical effects. The bases of mountains are mostly denuded by streams of water, their summits by streams of air. . The winds that sweep the jagged alps assume magnificent proportions, and effect changes of considerable importance. The smaller particles of disintegration are rolled or shoved to lower levels just as they are by water currents, or they are caught up bodily in strong, passionate gusts, and hurled against trees or -,M ] i874-] STUDIES IN THE SIERRA. 401 '\ fi*-/ higher portions of the surface. The manner in which exposed tree-trunks are thus wind-carved, will give some conception of the force, with which this agent moves-/-iOVl Where bowlders of a form fitted to shed off snow and rain have settled pro- tectingly upon a polished and striated surface, then the protected portion will, by the erosion and removal of the unprotected surface around it, finally come to form a pedestal for the stone which nitic region. They frequently wear a single pine, jauntily wind-slanted, like a feather in a cap, and a single large bowlder, poised by the receding ice- sheet, that often produces an impression of having been thus placed artificially, exciting the curiosity of the most apathetic mountaineer. Their occurrence always shows that the surfaces' they are resting upon are not yet deeply eroded. Ice-planed veins of quartz and feld- saved it. Fig. 2 shows where a bowlder, B, has settled upon and protected from erosion a portion of the original glaciated surface until the pedestal A has been formed, the height of which is of course the exact measure of the whole quantity of post-glacial denudation at that point. These bowlder pedestals, furnishing so admirable a means of gauging atmospheric erosion, occur throughout the middle granitic region in considerable numbers; some with their protecting : bowlders still poised in place, others /(X./ naked, their bowlders' having rolled off on account of the stool having been eroded until too small for them to balance upon. It is because of this simple action that all very old .ridges and slopes are boVyJderless, Nature having thus leisurely rolled them off, giving each a whirling impulse as it falls, from its pedestal once in hundreds' or thousands of years. Moutoneed rock forms shaped like Fig. 3 are abundant in the middle gra- FiG. 3- spar are frequently weathered into relief by the superior resistance they offer to erosion, but they seldom attain a greater height than three or four inches ere they become weather-cracked and lose their glacial polish, thus becoming useless as means of gauging denudation. Ice-burnished feldspar crystals are brought into relief in the same manner to the height of about an inch, and are available to this extent in determining denudation over large areas in the upper portion of the middle region. This brief survey of the various forces incessantly or occasionally at work wasting the Sierra surface would at first lead us to suppose that the sum total of the denudation must be enormous; but, on the contrary, so indestructible are the Sierra rocks, and so brief has been the period through which they have been exposed to these agents, that the general result is found to be comparatively insignificant. The unaltered polished V- \j OS°i3l 402 BILLY'S WIFE. [Nov. areas constituting so considerable a portion of the upper and middle regions, have not been denuded the one-hundredth part of an inch. Farther down measuring tablets abound bearing the signature of the ice. The amount of torrential and avalanchial denudation is also certainly estimated within narrow limits by measuring down from the unchanged glaciated surfaces lining their banks. Farther down the range, where the polished surfaces disappear, we may still reach a fair approximation by the height of pot-holes drilled into the walls of gorges, and by the forms of the bottoms of the valleys containing these gorges, an'cl by the shape and condition of the general features. Summing up these results, we find that the average quantity of post-glacial denudation in the upper half of the range, embracing a zone twenty-five or thirty miles wide, probably does not exceed a depth of three inches. That of the lower half has evidently been much greater — probably several feet—but certainly not so much as radically to alter any of its main features. In that portion of the range where [see study No. IV, in . the OvE*kAND.for August of this year] S-te/i/u-.cJluAf the depth of glacial denudation exceeds ;'. a mile, that of post-glacial denudation is W less than a foot. From its warm base to its cold summit, the physiognomy of the Sierra is still strictly glacial. Rivers have only traced shallow wrinkles, avalanches have made scars, and winds and rains have blurred it, but the change, as a whole, is not greater than that wh-ierv-eome's on a human countenance by aKfew years' j-L l Alpine storms. " '■ >"uj exposure to common Alpi - BILLY'S WIFE. DON'T know Billy? Then allow me to introduce to you "an Irishman by trade, and a mechanic by the grace of God;" a perfect pet among the girls, as handsome as the day is long, and as good as he is handsome. That is Billy, or rather was Billy when first I knew him. He was the youngest of the squad of carpenters that worked on our new building, and altogether the most active and efficient of the party. They sent him to all the exposed positions, and he risked his life gayly every day. "There is a special Providence for fools and children" said he, "and I claim protection under the first clause of that law." I was watching him one day at work on the top of the house, when his foot slipped, and he started head foremost for terra firma. When within ten feet of the bottom he touched some scaffold ing with his hand, "swapped ends" and lighted on his feet, like the tumbler in a circus, made me an elaborate salaam, and climbed up again to his eyrie full forty feet above my head. Billy had https://scholarlycommons.pacific.edu/jmb/1084/thumbnail.jpg