transverse stream and drain them off to a lower. This process was repeated so often, that the transverse streams dwindled, the subsequent streams grew at their expense, and the longitudinal valleys became, as they are today, the prominent and impressive erosional features of the region. The full development of such a system of subsequent drainage is a characteristic of an old geomorphy. The courses of the streams have long since passed under the control of the underlying structure of the country, and they are fully adjusted to that structure. In contrast with this, the drainage of the Sierra Nevada is adjusted merely to the original slope of the surface of the range and is almost entirely independent of the underlying structure. The feebleness of the subsequent drainage and the dominance of the consequent cañons is a mark of geomorphic youth, and, although the range has been degraded to nearly one half its original volume, and although the corrasion of the vast cañons by the streams must have taken a very long period of time, measured in years or centuries, yet, from a comparative point of view, the Sierra Nevada may be safely placed among the young and giddy mountains of our planet.

This conclusion is confirmed by a consideration of the eastern front of the range overlooking the Great Basin. This, as has been stated, is a somewhat degraded fault scarp, originally the exposed side of the block along the plane of its dislocation from the country to the east. The scarp, under the disintegrating action of the atmosphere, has been undergoing a slow recession ever since its emergence. This disintegration is probably promoted by gravitational stresses within the mass, which find relief on uplift in the disruption of the rocky mass along systems of intersecting parting planes known as joints. The blocks bounded by these joints are readily dislodged by the heaving action of frost, and innumerable such blocks have been shed from the face of the fault scarp,

thus effecting its gradual recession. Owing to the greater rapidity with which this process proceeds in the upper portion of the scarp, and the protection afforded to the lower portion by the accumulating talus of fragments that fall from above, the tendency is to reduce the original declivity to a more moderate slope. In a geological sense this degradation of the originally steep face of the scarp is a rapid process. Yet how little has it accomplished! To stand on the brink of Mt. Whitney, to view the great Sierran wall to the north, to the south, to gaze down into Owen's Valley over two miles below, to look out over the vast expanse of "the land of little rain" as from a balloon, to stand there is to drink in one of the most impressive lessons in American geologyand that lesson is the recency of the upheaval of the Sierran block. The great wall has suffered but little from the vigorous and incessant onslaught of the forces of degradation. It is still a wall. Even along those lines of maximum intensity of the erosive process, the stream courses, the results are small. The steep grade of these easterly streams suggests that they should in time have cut through the general line of the crest and thrown the divide far to the west of it. The erosive power of streams is a function of their grade, and it is a well established principle in geology that if the streams on two sides of a mountain ridge be of unequal grade the steeper will have the advantage, and, other things being equal, will cut their cañons back into the ridge faster than those flowing down the gentler slope, and thus bring about a gradual migration of the divide down the gentler slope till equilibrium be established. This migration of the divide between the east and the west flowing streams in the Sierra Nevada has made but little progress. This is due partly to the fact that other things are not equal. The supply of water, for example, from the snows of the crest region is much more abundant for the westerly streams than for the easterly. This compensates in a

measure the difference in grade, but it could not preclude the steeper grade streams from eventually cutting through the crest and drawing their headwaters from the western slopes at the expense of the westerly streams. As a matter of fact this migration of the divide to the west of the crest has already been accomplished and it is only to the smallness of that migration, the slight approach which has as yet been made towards equilibrium, that attention is directed. The fact that the divide so nearly coincides with the brink of the range is an additional proof of the youthfulness of the Sierra Nevada.

III

The various approaches to the High Sierra from the Great Valley tend to follow the divides between the cañons of the westerly flowing streams. Many of these divides in the middle slopes of the range are by no means sharp ridges, but are flat or undulating surfaces of considerable breadth. If one stands on a commanding point upon one of these broad divides, where the outlook is unobscured by timber, he usually enjoys an astonishing sweep of view. He can look down the slope of the range into the Great Valley, and above the valley haze he can see the outlines of the Coast Range. He can look up the slope of the range and see in the distance the crisp profile of the snow flecked crest against a flawless sky. He can look far to the south and far to the north, and there is no ridge higher than that on which he stands to cut off his view. Ridge after ridge falls away in a broad perspective at nearly uniform levels. He loses sight of the intervening cañons and, if he have even a feeble imagination, the total effect is that of an expansive plain, a sloping plain, to be sure, and a not very even plain, but nevertheless a plain-the antithesis of a mountain landscape. If our observer knows that the intervening cañons between the great ridges are the products of

stream erosion, he may, in spite of his feeble imagination, suppose for a moment that they have been filled up again flush with the tops of the ridges. If he succeed in this, and further suppose the distinct westerly slope of the range to be greatly subdued, he will have before his mind's eye the essential features of the region as they existed prior to its dislocation and tilting.

This ancient surface, as it was before the Sierra Nevada mountain block was evolved from it by upheaval, is known technically as a peneplain, a term introduced by Professor Davis, of Harvard, to designate erosional surfaces too uneven to be classed as true plains.

Now, this recognition of the former condition of the Sierra Nevada region as that of a peneplain leads to further interesting considerations. A peneplain of the extent of that with which we have here to deal, is the penultimate stage in a long continued degradation of the region by atmospheric and fluviatile erosion. At this stage the streams meandered in broad flood plains in relatively shallow valleys, separated by low, tortuous divides. The streams, long before this stage was reached, had ceased to cut downward and had established the lowest possible grade to the sea into which they discharged. It follows from this that the peneplain, which we have pictured to ourselves as occupying the Sierra Nevadan region prior to its uplift, lay at but small elevation above sea-level.

But a peneplain may, toward the headwaters of the streams which traverse it, pass into a hilly country; and there is reason to believe that, to the east of this peneplain, there was a mature, well rounded range of hills, having a maximum altitude of about 3000 feet above sealevel, which served as the divide for the waters draining across this peneplain to the Pacific. A part of the summit region of the Sierra Nevada includes remnants of this hilly range, but it is improbable that it paralleled the present crest line.

It is to be noted, moreover, that, in the reduction of such a region to the peneplain condition, masses of exceptionally hard rock would be reduced more slowly than the rest of the region, and might be left standing upon the peneplain as rather prominent residuals after the rest of the region had become one of subdued relief. Such residuals are well known on the slope of the Sierra Nevada. Banner Hill, near Grass Valley, may be mentioned as a typical instance of this sort of feature.

IV

Now this peneplain standing at low level, with its meandering streams, its vague divides, its occasional residual hills and its distant well rounded range at the source of its streams, has been stated to be the penultimate stage in the degradation of the region. This means that the region had been above sea-level and subject to the attack of erosional forces for a long period of geological time. During this period a preëxisting mountain mass, totally distinct from the present Sierra Nevada except in general geographic location, had been almost entirely obliterated. The peneplain was but the abraded stump of this vast mass. In the stump we can read its structure and general history. Its mountain structure was of a very different type from that of the present Sierra Nevada. It had more the character of the Himalaya or the Alps, and its upheaval was accompanied by, if not caused by, the intrusion of a vast body of molten granite in its central part. It was, however, only a local manifestation of a general orogenic disturbance which affected the greater part of the Cordilleran region from the Wasatch westward. The rocks which were invaded by this body of molten rock welling up from unknown sources in the deeper portions of the earth's crust, were ordinary sedimentary and volcanic formations which had been accumulating over the region in nearly horizontal