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Show 104 of country that were entered. Almost the entire region included between " the Plains" and the Pacific Ocean is mountainous ; that is, the rocks which are its foundation are bent aud broken and uplifted into ridges, which ridges are mountain ranges. But there is one exceptional area in which the beds of rock lie level, or nearly so, and this was named by myself in the year 1871 the " Colorado Plateau" region. In its general features, it contrasts very strongly with the surrounding mountain country. It consists of a system of tables, in places rising above each other in step- like order, and elsewhere divided by narrow, deep, and often impassable gorges. A large portion of this region is drained by the Colorado and its tributaries, but other parts send their waters to the Sevier Desert and via the Rio Pecos to the Rio Grande. Of the political divisions, it comprises portions of Utah, Colorado, Arizona, and $ ew Mexico, and its physical boundaries are the Uintah Mountains at the north, the Rocky Mountains at the east, and at the west and south the regions of the Cordilleras. Through the labors of the topographers and geologists, the general dimensions of this area are now for the first time known, and its western, southern, and southeastern boundaries, making a line 900 miles long, are drawn on the map. Within it are exceptional opportunities for the study of certain special geological subjects, and to these their attention has been turned. One of them is that of erosion by running water, which finds its superlative expression in the canons of the Colorado, and is there contrasted with erosion by rain and compared with erosion by drifting sand. Another subject is that of the origin of mountains; for, although the plateaus are distinctively not mountains, they contaiu certain simple dislocations which are the germs of mountains, and bear the same relation to dynamical geology that embryos bear to biology. These dislocations are faults and simple folds, and they have been traced and studied for hundreds of miles. Another subject to which great attention has perforce been given is that of volcanic phenomena. Every State aiid Territory west of the plains, every physical division the Rocky Mountains, the Plateaux, the Cordilleras, are crowded with the products of volcanic action, ancient and modern. Hardly a mountain range lacks them. The largest consecutive areas without them are among the plateaux; but in that same province, also, are some of the largest lava- fields. In Southern Utah there are connected floods of lava, covering an area of 5,000 square miles; and of this area the geologists of the expedition have obtained data for geological maps. They have also approximately defined the limits of a similar area in Arizona and New Mexico not less than 20,000 square miles in extent, and never before recognized as a connected belt. Of the conclusions which they draw from their accumulation of volcanic data, one, at least, is of general interest, namely: that eruptions in our western territory will be again resumed, which occurrences may take place at any time. In the past, they have occurred so recently that it is, indeed, surprising that there is no human record of them* The distribution of the geological formations has been made out with a good degree of accuracy in the portions of the plateau country and Rocky Mountains that have been examined; but in the region of the Cordilleras less has been accomplished, for the reason that all relations there are more complicated and no strata can be traced continuously for great distances. Among the contributions to stratigraphical knowledge are the determination of the Tertiary age of the Sam Pitch coals of |