attitudes throughout Paleozoic and the early half of Mesozoic time; the invasion having occurred at the close of the Jurassic. This granite probably never appeared at the surface as such, but consolidated under a covering or roof of these sedimentary rocks. Portions of this covering of stratified rocks, in a flexed and closely appressed condition, sank deeply into the molten mass and are preserved to us in that relationship to the granite to this day. In other cases the contact surface, between the molten granite and the roof of stratified rocks, maintained a more nearly horizontal, or at least a simple undulatory, attitude. In the vicinity of their contact with the granite, the sedimentary rocks are profoundly altered from their original condition; yet in spite of this metamorphosis, they have yielded sufficient fossils to establish their geological age. The granite cuts across various stratigraphic horizons in the sedimentary series, but, in the hundreds of miles along which the contact has been traced, in a territory of many thousands of square miles, the basement or floor upon which the series as a whole once rested has nowhere been found. It has been displaced by the granite, whether by resorption in the hot magma or by sinking into its depths, is not yet known. In this phase of our study of the Sierra Nevada we are thus introduced to one of the most profound and most mysterious problems in geology, viz.: the genesis and behavior of granitic magmas in the earth's crust.

V

The granitic rocks and the stratified rocks into which they were intruded, in so far as these are left to us by erosion, constitute what is known as the Bedrock complex. Before the abraded stump of this complex was tilted to form the present Sierra Nevada block, however, another series of formations had been spread upon its surface. These are known as the superjacent series.

These formations lie in a general way parallel to the slope of the Sierra Nevada, having participated in the tilting of the block. They have been dissected by the great cañons which have sunk into the Sierra Nevada in consequence of its uplift. They are therefore known to us only in remnants lying upon the broad plateaulike divides between the cañons, but in the lower foothills of the range, along the eastern margin of the Great Valley, they form a fairly continuous sheet dipping at a low angle beneath the alluvium of the valley floor. The earliest of these superjacent formations are river gravels, which accumulated in broad shallow valley bottoms and are, par excellence, the famous, auriferous gravels or placers of the Sierra Nevada. These were buried by a series of volcanic mud flows, tuffs, agglomerates, and lavas, which, after filling up the valleys in which the gravels lay, finally spread out over the entire northern half of the region, levelling up the inequalities of the peneplain and transforming it into a perfect plain. This process of volcanic accumulation was, however, an intermittent one, and in the intervals between the different volcanic deposits, streams flowed through shallow valleys and spread out their flood plain gravels, which were in turn buried by succeeding showers of volcanic ashes or lavas. In these intervals the region was clothed with a luxuriant semitropical vegetation, as the abundant fossil leaves found in the finer deposits and the fossil wood found in the gravels amply testify. The formations of the superjacent series were laid down on a land surface, the only waters concerned in their deposition being those of the rivers; but on the western margin of the region, along the edge of the Great Valley, these same subaerial formations pass into deposits which were laid down in bodies of water, probably esturine and marshy in character, for here we find sandstones, and clays, and beds of coal; and these are apparently the equivalents of certain well defined marine formations of the Coast Ranges which are known to be of early Tertiary age.

VI

We have thus far by convenient approaches and roundabout paths, as is the rule in finding one's way through the mountains, come upon some of the more important facts in the evolution of the Sierra Nevada as we know them today. It will be of advantage, perhaps, by way of summary, to arrange these briefly in their historical order. We have to begin with the Paleozoic and early Mesozoic marine sedimentary rocks and their intercalations of contemporaneous volcanic accumulation, for nothing earlier is positively known to us. These have within themselves the records of a prolonged and by no means monotonous history, which cannot here even be touched upon. But, treating these formations as a whole, it may be safely said that they at one time extended over the whole of the Sierran region. In middle Mesozoic time these formations were profoundly affected by what may properly be called a convulsion of the earth's crust. The strata were doubled up on themselves, sheared and otherwise intensely deformed, and were, as a final result of this disturbance, invaded by a mass of molten granite, which, beneath the surface, was far more extensive than the Sierra Nevada Range. As a result of this, the sedimentary and volcanic rocks were metamorphosed in two distinct ways, first, by the internal activities generated in them by their deformation, and, secondly, by the heat given off from the intrusive granite. The gold-bearing quartz veins of the range are also genetically connected with this intrusion. The general result of this disturbance was a mountain mass of Himalayan or Alpine type. Since then the region has never been submerged beneath the sea; in this respect it is in striking contrast to the neighboring Coast Range region, which, in the same interval of time, has been repeatedly depressed beneath the waters of the ocean, and has as often emerged. Having been always above sea-level,

this mountain mass has been continuously in the zone of erosion, and by early Tertiary time it had been degraded to a peneplain, the drainage of which was close to sea-level. The auriferous gravels accumulated in the shallow valleys of this peneplain, and these in the northern half of the region, at least, were buried by accumulations of volcanic tuffs, later gravels, agglomerates, and lavas. Then ensued the dislocation and upheaval of the great mountain block by at least two distinct and widely separated movements. By these movements the great scarp on the eastern margin of the range was formed, the streams were changed in their courses, their erosive power was rejuvenated, and the scoring of the Sierran cañons inaugurated. The second of the two elevatory movements concerned in the uplift of the range, and possibly the first movement also, was accompanied by eruptions from a series of volcanoes distributed along the line of dislocation at the base of the great scarp. A few small volcanoes have been built up at a much later date within the mountain valleys.

VII

There remains to be noted one other important event in the geological history of the Sierra Nevada, and that is its glaciation. This occurred in the geological yesterday. After the dissection of the uplifted block had proceeded so far that the Sierran cañons had attained their present depth, and the geomorphic development of the middle and lower slopes of the range was much as it is today, the climate of the region changed, so that the warmth of summer was unable to remove the winter snows of the summit region. Whether this change involved a diminution of the mean annual temperature or merely a notable increase in the winter precipitation is a debatable question, which need not be entered into here. Certain it is that the snows accumulated through the

years and centuries till by their weight they consolidated into ice, and as such flowed down the cañons for many miles from the crest to the west, and for a much shorter distance down the steep eastern scarp toward the Great Basin. This ice did not, however, entirely mantle the crest region. All the higher peaks and ridges rose above the gathering ground of the ice. The major centers of snow accumulation and, therefore, of ice distribution were located at the heads of the cañons and in the lee of the ridges and peaks, where conditions for drifting were favorable. As these ice streams moved slowly down the cañons, the latter were greatly modified. The rocky débris which mantled their slopes was caught up by the ice and carried forward. This material and similar fragments plucked by the ice from the rocky surfaces with which it came in contact, served as graving tools, and the floors of the cañons were deepened and widened by abrasion. In this way the normal V-shaped profile of the stream cañons was changed to the characteristic U-shaped profile of a glacial channel. But it was at the very sources of these ice streams that the most remarkable transformation of the form of the mountains was effected. Here the ice had the power of eating its way into the heart of a great mountain ridge or peak by a process of sapping the rocks. This process caused the head of the glacier to become half encircled by precipitous walls, although the floor upon which the ice rested was comparatively flat. By the recession of these encircling walls, under the sapping process at their base, great bite-like amphitheatres, or cirques, were carved out of the high crests. Since several ice streams, flowing in different directions, usually headed in one ridge, it frequently happened that opposing cirques met, and by the intersection of their precipitous walls effected an amazing reduction of the altitude of the summits in a comparatively short time, leaving a mere knife edge between the cirques, 1000 or 2000 feet below the original