PEEPATORY NOTE. xi 



many outflows, which are often separated by unconformities due to inter- 

 vening 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 charac- 

 teristics 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 com- 

 posed of sedimentary formations of eai-ly Tertiary and late Cretaceous age, 

 nearly or quite horizontal and usually capped with formations of extrava- 

 sated matter. These lavas are of exceedingly complex arrangement. The 

 period of volcanic activity was long, and between the outbreaks atmos- 

 pheric 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 chffs 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 



