Page:Popular Science Monthly Volume 18.djvu/865

Rh the south-southwest, and having a breadth of from twenty-five to eighty miles, and a total area approaching nine thousand square miles. They are distinct, structurally and topographically, from the Wahsatch range, belong to another age, and are wholly different in their forms and geological relations. They are composed of early Tertiary and late Cretaceous formations, nearly or quite horizontal, and usually capped with lava formations of exceedingly complex arrangement. The region is for the most part destitute of vegetation and soil, and dissected by deep cañons. Consequently, its geology, as a whole, is plainly revealed, so that every fault, every flexure, the relations of successive unconformities, and all facts of structure are seen at once; but two sources of obscurity exist, in that some of the highest plateaus are covered with forests and vegetation, and that the extravasated rocks are aggregated in a more confused manner than the sedimentary beds, so that uncertainties and doubts still remain after the utmost labor and care in tracing them. The surface of the plateaus appears to have formed the bottom of a lake in Eocene times, and to have gradually risen to its present height by a movement which may still be going on. The drainage is by the tributaries of the Colorado, whose channels were formed in the lake-bottom before it was wholly dry, and have kept their course and level where they are, "in spite of faults, flexures, and swells, in spite of mountains and plateaus," the streams turning neither to the right nor to the left as these irregularities were encountered, but persistently cutting their way through the same old places, till the present magnificent cañons have been carved out. Another salient feature of the region is given by the extraordinarily extensive faults, the results of displacements which took place in relatively recent times. Some five or six of these great displacements are from twenty to a hundred miles long, and are of heights rising to a maximum of five thousand feet. One of them, the Eastern Kaibab fault, is the longest line of displacement of which the author has any knowledge; it has a length which can not fall much short of three hundred miles, and may be found to exceed that after its termini have been discovered, and a maximum height of seven thousand feet. The displacements do not belong wholly to any one period, but are of distinct though not widely separated ages. The erosions of the plateau do not appear to have been affected by the presence of ice during the glacial epoch, but the evidence is strongly in favor of the conclusion that the climate in this district was not glacial. "The ravines and valleys are conspicuously water-carved, and conspicuously not ice carved." Yet evidences of the former existence of small local glaciers are found on the summits of the cliffs, not less than eight thousand five hundred to nine thousand feet high; and this is believed to emphasize the evidence of the absence of ice-action in the valleys and plateau flanks.

The region is one of extinct volcanism, the action of which, though small compared with that we know of some other regions, has been great compared with what is seen in most of the volcanic districts of Europe. The phenomena are of the most varied kinds, and relate to eruptions of which the oldest go back to the middle Eocene, while the latest "can not be as old as the Christian era," and "it is hard to believe that they are as old as the conquest of Mexico by Cortes." They are subjected to a careful study, especially with reference to the order of succession of the eruptions, and a comparison of the same with the arrangement proposed by Baron Richtofen. This study is followed by a discussion of the origin of volcanic eruptions, as illustrated by these phenomena. The photographic work not having been completed, has not been embodied in the present volume. The text is more especially devoted to the general geology, while many of the details are made more clear by heliotypic illustrations than they can be by mere textual descriptions. The atlas contains topographical, geological, and relief maps, and a sheet showing the arrangement of the faults and flexures.

latest study of plant-life shows no abatement of his power of work or his habits of fresh and original observation. We have learned to expect from him I at intervals, never much prolonged, the