OCR Text |
Show PREFATORY NOTE. xi many outflows, which are often separated by unconformities due to intervening atmospheric degradation. In the third, the mountains are composed in part of sedimentary and in part of extravasated materials. The sedimentary beds constitute the central masses, over which extravasated rocks are spread. The locus of extravasation being above the general base level of degradation, as the adjacent country was carried away by atmospheric agencies the underlying sedimentaries were protected and left as mountain masses. Usually the extravasation has been continued from time to time through a series of vents marked by cinder cones, and in a general way the earlier ones appear nearer the summit of the mountain masses, the later ones nearer the base. In this manner the several sheets are inversely imbricated; that is, the upper edge of the lower sheet is placed on the lower edge of the upper sheet. " Table Mountains," with caps of lava, are the simplest forms of this structure. There are many varieties of each of these grand classes, and through them the systems of structure coalesce in such a manner that the characteristics of demarkation are not absolute. The Colorado Plateaus may be divided into a number of groups, based on topographic and geologic characteristics, of which the High Plateaus constitute one of the most important. The great tabular masses are composed of sedimentary formations of early Tertiary and late Cretaceous age, nearly or quite horizontal and usually capped with formations of extravasated matter. These lavas are of exceedingly complex arrangement. The period of volcanic activity was long, and between the -outbreaks atmospheric degradation, local transportation, and deposition intervened. To unravel these complexities and discover the line of sequence has been a task of great magnitude. In the earlier explorations of this country under the direction of the writer, the general sequence of sedimentary formations was discovered, as well as the general characteristics of displacement, many of its principal faults had been traced, and the origin of the cliffs and canons was known. All this was the result of a series of reconnaissance surveys. But the principal work of the geological survey of the region still awaited accomplishment. It was necessary that the sedimentary formations |