stream, what manner of rocks might be expected in the moraines and what particular branch of the glacier had transported them. On the other hand, it sometimes happened that the physiographer, upon entering a new district, would notice in the moraines fragments of a new rock type, and thus was able to predict the existence of a certain formation in the region whence the ice had come. The two men thus carried on team work of a mutually profitable sort. That the identification of the disintegrated older moraines was not always an easy matter, may readily be surmised. The rocks of the Yosemite region unfortunately are almost exclusively granites, popularly speaking, that is, crystalline igneous rocks closely allied to each other and not greatly differing in general aspect, at least not to the untrained eye. When thoroughly weathered these rocks look much alike. As a consequence the physiographer was at times obliged to stop his pacing and spend an hour with his geological hammer breaking pebbles and chipping corners off boulders in order to discover one of the desired diagnostic types. Such work, however, although it consumed much time, usually yielded gratifying results. An afternoon of rock breaking on Glacier Point established once and for all the fact that that promonotory has been overwhelmed by the ice of an early epoch-an epoch so early that except for some boulders and cobbles encased in residual soil in a protecting hollow, all sign of the ice flood has long since disappeared. Yet the evidence is indisputable, many of the cobbles unearthed and broken proving to belong to rock types from the Little Yosemite region and foreign to the Glacier Point neighborhood. It may be said in passing that the basin-shaped hollows in the bare rock surfaces of Glacier Point are not pot-holes produced by streams under the ice, but are merely the result of rapid disintegration of the rock in particularly vulnerable spots. It is certain that the ice did not leave these depressions, for there is ample evidence in various localities showing that since the early ice flood which visited Glacier Point the surface of the rock has been removed to the depth of several feet from bare, unprotected places of this sort by the slow processes of disintegration. One thing must have become patent from the foregoing pages, namely that the study of a glacial valley such as the Yosemite can scarcely hope to be exhaustive unless it be extended to the higher regions that were the sources of the ice. The need of an acquaintance with these regions soon impressed itself upon the investigators and accordingly they deployed over the entire headwaters country of the Merced River as well as over the basin of the Tuolumne. The latter territory proved of unusual interest, for it evidently constituted the great central névé field of the middle Sierra and poured its ice not only down the Grand Cañon of the Tuolumne, but across several passes into the Tenaya and Merced basins, as well as down the cañons of the east facing Sierra front. The height attained by the ice and the extent of its various diversions during each separate epoch were ascertained with considerable care, while at the same time the rock types present in the area and transported from it by the ice were examined in some detail. Late in September the investigators returned to the Yosemite Valley, and resumed their labors there enriched with an insight into the character of the region that was the birthplace of the Yosemite glaciers. A visit was also made to the Hetch Hetchy Valley, the Grand Cañon of the Tuolumne and Lake Eleanor. Some valuable lessons in the glacial history of the Sierra Nevada were learnt on that trip, not the least valuable of which was derived from the fact that the ice stream that traversed the Hetch Hetchy Valley was vastly superior in volume and in length to the Yosemite Glacier. The opinion has been advanced that the lesser dimensions of the Hetch Hetchy Valley bespeak erosion by a less powerful ice stream than that which enlarged the Yosemite Valley. This is utterly refuted by the morainal record. The Hetch Hetchy Valley, it appears, was fairly smothered under ice at times when the Yosemite was but half filled. That it was not enlarged proportionately to the volume of its glacier is to be ascribed primarily to the exceeding resistance of its massive rocks. The lesson is that in a region of massive granites such as the Sierra Nevada, the size of a valley cannot safely be taken as an index of the magnitude of the glacier that passed through it. This brings us to that part of the investigation, the aim of which was to determine the amount of erosional work performed by the ice. On a previous page, it will be remembered, the statement was made that the sculpture of the Yosemite Valley is erratic and, as an index to ice erosion, most deceiving. The explanation lies, for the Yosemite as it does for the Hetch Hetchy, in the structural characteristics of the country rock. The interpretation of these characteristics given in the recent booklet on the origin of the Yosemite Valley, published by the Department of the Interior, the writer is happy to state, was verified and confirmed by the structural studies of last summer. As was pointed out in that booklet, the granites of the Yosemite region are peculiar in that they are not everywhere traversed by natural partings or "joints." Large masses of them, some half a mile or more in extent, have remained undivided, absolutely solid, although in other places the same rock may be fissured at intervals of only a few feet. That these extreme variations in structure must have greatly affected the eroding efficiency of the ice, will readily be understood. While the glaciers might work to advantage in rocks divided into small joint blocks, they were relatively powerless when dealing with massive granite. The latter they could reduce only by superficial abrasion, a slow process the efficiency of which has been much overestimated. As a result, the ice-carved topography of the Yosemite country is a singularly varied one, well developed, typical glacial forms alternating with others not suggestive of ice work at all. On the whole, it may be said that this phase of the Yosemite problem has remained the most misunderstood. Former observers have not always made sufficient allowance for the exceptional nature of the granites of the Sierra. Perhaps they had in mind the work of glaciers in regions of more normally jointed rocks and thus were misled in assigning too great erosional achievements to the ice streams of the Yosemite region. The main lessons learned last summer in this regard are that except in certain restricted localities, such as the Yosemite Valley proper, the rock character precluded extensive remodeling by the ice, and that as a consequence the landscape, although bearing the unmistakable stamp of ice |