The Black Hills have been referred to as originating by a simple local doming of the earth's outer crust. But the dome, for the greater part of its profile in all azimuths, has long since vanished. It has been removed by erosion. Out of the original dome there has been evolved an oval shaped area of hilly country girdled more or less continuously by inward facing escarpments. In the popular mind the area is occupied by an intricate group of unrelated hills, as the name implies. To the geologist these hills are but the residuals of a mountain unit of a very simple type; and in systematic orography the Black Hills are classified, not on the basis of the resulting degradational forms, which express but the details of the structure as revealed by erosion, but upon the basis of the primitive form, that of a dome-now almost destroyed. The great arch of the Uinta no longer exists. The highest peaks of the range fall far short of the altitude of the original arched surface. The vast rounded ridge has been pared down just as in the case of the Black Hills dome; and in the present topography we are dealing with the erosional residuals. But the geologist, from his studies of the structure revealed in these residuals, restores and mentally reconstructs the simple arch due to formative upheaval, and upon that basis recognizes the unity of the apparently chaotic upland, classifies the mountain, and compares it with others.

Similarly the imposing ridges of the Jura and the Appalachians are not the crests of the original folds. They are the protruding edges of belts of hard rocks exposed in the course of the degradation of the mountains. These belts of hard rocks appear on the flanks of the folds and are separated by longitudinal erosional valleys which follow the parallel belts of soft rock. The character, disposition, extent, and multiplicity of these folds having been inferred from the study of the strata exposed in these ridges and valleys, that inference becomes as before the basis of the classification of the mountains.

From the various cases cited it must be apparent that, while the degradational forms may be matters of primary interest to the physiographer, it is the general structure of the original block from which such forms have been carved that claims the attention of the geologist in his attempt to study mountains, to discriminate their chief types, and so make some advance toward solving the underlying problem of their genesis. It is the general structural plan which he seeks to decipher in all cases, and in this search he generally finds a unity to which the popular eye is blind. Except in the case of living or recently extinct volcanoes he has usually to deal with the ruins of mountains. And, just as the archaeologist or architect works out the original plan of an ancient edifice from a study of its ruins, and classes it as belonging to this or the other style of architecture, so the geologist, from a consideration of the wreck of a mountain, is able to picture to himself the structure and configuration it would have possessed if unaffected by degradational forces, and thus determines to which of the various types of mountains it belongs.

II

Now, the Sierra Nevada is, in the sense just indicated, of the Basin Range type of mountains. It constitutes a vast block of the earth's crust, many times longer than broad, which has been rotated on an axis parallel with its length, so that the eastern edge, dislocated from the region of the present Great Basin, has been elevated to make the crest of the range, while the western edge has been depressed below sea-level and is buried by the deposits which underlie the floor of the Great Valley of California. This is a very simple conception of the general structure and configuration of what appeals to most mountaineers as a veritable "sea of mountains," as they view the Sierra from some of its

commanding peaks. It substitutes unity for plurality, although we are still constrained to refer to the range in the plural. The unit in this case is large, even as mountains go, and the Sierra Nevada may be regarded as the largest as well as the finest example of simple tilted blocks of the Basin Range type, so far as our authentic information at present indicates.

The bewildering "sea of mountains" which one looks down upon from the peaks of the high Sierra are the result of the sculpture of this great block. The disintegrating action of the atmosphere, the stress of gravity, the transporting action of running water, the corrasive action of the load of rock débris as it moves forward in the streams, the corrading, sapping, and transporting action of glaciers-these have all conspired to carve out of the Sierran block the magnificent features which we designate, collectively, the mountains. Ever since the inauguration of its uplift, the mountain mass has been subject to the ceaseless attack of some or all of these degrading forces. They have evolved the profound cañons which dissect it and out through these cañons there has been swept probably half the original volume of the tilted block. We might picture to ourselves the original form of the block, in its simplest outline, by imagining these great cañons filled up to the level of the great ridges which divide them one from another. The picture thus formed, however, by the effort of the imagination, is not a true picture of the mountain at any stage of its evolution; it is rather an outline of the tilted block as it would be today had it never been affected by the degrading forces of erosion. But these forces have been at work since the very inception of the elevatory movement. This movement has been a slow and intermittent one, so that the dissection of the block and the general degradation of its surface have proceeded hand in hand with its gradual uplift. From these considerations it appears that the conception of the Sierra

Nevada as a tilted block of the earth's crust of simple asymmetric profile, is one which has never been realized in fact. The abstraction which the geologist makes use of for the purpose of determining the type of mountain is confined to the result of the formative process and ignores the degradational process. But while we can thus think clearly of the one process independently of the other, in reality the product of the formative uplift has never for a moment been unmodified by the process which makes for obliteration. It is an interesting illustration of the subsurface idealism which pervades nearly all scientific thought.

It is interesting to note the control which has been exercised by the formative movement upon the directions along which the degradational forces have worked with maximum intensity. These directions are indicated by the drainage scheme of the range. By the rotation of the rising Sierran block its eastern edge became the crest of the range and the divide for the drainage. To the east of that edge there was a precipitous fault scarp and the streams on that side had a short run with very steep grade to the Great Basin. To the west of the same edge there was a long, comparatively gentle slope to the Great Valley, and the westward streams first began the downward corrasion of the present Sierran cañons by flowing across this slope. The drainage system of the ancient surface prior to the tilting was rearranged to conform to the now dominant slope. The new drainage which thus arose in consequence of the tilting of the surface of the block is known as a consequent drainage, and this consequent character of the streams of the Sierra Nevada persists to this day. The divide is still at the eastern brink of the tilted block and from it the streams, with one or two exceptions, which have special explanations, flow westerly, or directly across the axis of the block, scoring profound cañons through hard and soft rocks alike without significant deviation. In the sinking of

these cañons, some streams had an advantage over others by reason of the larger volume of water, so that the downward corrasion proceeded faster, and the less powerful of the original consequent streams were forced to drain into these vigorous ones, thus becoming tributary to them and adding to their vigor. In this way the original number of trunk or through drainage lines has been greatly reduced; and in this way we may explain the ramifying character of the drainage lines about the upper stretches of the main streams. Other tributaries may be remnants of the streams that antedate the uplift. But, besides these tributaries of early origin, we might reasonably expect that others would arise on the slopes of the great cañons which would be in no sense consequent upon the uplift, but which would be subsequent to the corrasion of the cañons. Such streams do exist in the Sierra Nevada, but they are all short and insignificant. Even where they run on the belts of soft rock, where they might be expected in time to have cut through the great transverse ridges and have entrenched themselves in notable cañons following the general trend of the range, we find that but little work has been done. When we compare the Sierra Nevada in this respect with the Appalachians, which originated at a much earlier date in geological history, we find in the latter a very pronounced development of these subsequent streams. The cañons and valleys eroded in the belts of soft rocks, and so running parallel with the trend of the range, are the dominant features of Appalachian geomorphy; while the transverse streams are relatively insignificant, being few and short. The reason for this is that the subsequent streams, being once established in the belts of soft rock, have been able to cut down more rapidly than the transverse streams, which had to flow over hard and soft rocks alike. This enabled the subsequent streams to cut through the ridges separating the transverse valleys and to intercept or capture the waters of a higher