C. A. White — Lower Cretaceous of the Southwest. 443 



that the latest stiata of the Comanche series are considerably 

 older than the earliest ones of the Dakota Group, but we yet 

 know of no formation, either upon this continent or elsewhere, 

 which can be confidently assigned to a place between them. 

 Between the Lower Cretaceous of the Southwest and the 

 underlying formations there is a distinct hiatus which w x ill 

 presently be further mentioned. 



In Texas, east of the Pecos river, and in Arkansas and the 

 Indian Territory, the Comanche or Lower Cretaceous strata, 

 while they are frequently in the condition of ordinary firm 

 limestones, often consist of friable, more or less concretionary 

 calcareous layers, the prevailing color of the whole being light 

 gray. All the strata are usually fossiliferous, often abundantly 

 so, and it is from the less compacted layers that most of the col- 

 lections of fossils have been made. Even where these eastern 

 Comanche strata are most displaced, they rarely consist of 

 densely compacted layers. 



East of Pecos river they are found on the east, south and 

 west sides of the Texas Paleozoic area, where the series ranges 

 in thickness from one hundred to eight hundred feet, with a 

 considerable thickening above this in the neighborhood of 

 Austin. Upon the western side of that area the whole series 

 disappears from view by a westerly dip ; and when it is brought 

 up again west of the Pecos in Texas, and south of the Rio 

 Grande in Mexico, by the mountain uplifts which are presently 

 to be further mentioned, it is found to have become greatly 

 thickened and its strata all changed to compact bluish limestone 

 closely resembling those of Paleozoic age, especially those of 

 the Carboniferous of the great interior region. In these 

 respects they are in strong contrast with all other known strata 

 of the North American Cretaceous. 



Within the region that includes the portion of Western 

 Texas which lies west of Pecos river, together with the ad- 

 jacent parts of the Mexican states of Chihuahua and Coahuila, 

 there are numerous clusters and short ranges of mountains, a 

 part of which are composed of erupted rocks, but many of 

 which are true orogenic uplifts of stratified formations. It is 

 the latter that have just been briefly referred to. These uplifts 

 have brought to view strata of various ae-es including Lower 

 Cretaceous, Carboniferous, Silurian and apparently still older 

 strata. The Upper Cretaceous, including the Laramie, was 

 also involved in the same displacements. Besides these strati- 

 fied rocks, a crystalline granitic rock is sometimes observable 

 beneath them, in the mountain ranges, which has sometimes 

 the appearance of having been intruded, but it is not thought 

 to have any relation to the erupted rocks which have been 

 mentioned as composing a part of the mountain ranges of this 

 region. 



