C. A. White — Lower Cretaceous of the Southwest. 441 



central Kansas, none have been discovered farther north 

 than the southern part of Indian Territory and the south- 

 western part of Arkansas. 



The results of observations made by myself during the past 

 season indicate that they do not extend so far northward in 

 New Mexico as the central part of that territory, although 

 they are known to exist in the extreme southern part. Several 

 years ago M. Remond obtained a collection of fossils contain- 

 ing characteristic species of the Lower Cretaceous from " the' 

 Sierra de las Conchas, near Arivechi, Sonora," which were 

 described and figured by Mr. Gabb in volume ii, Paleontology 

 of California. This is the most westerly locality at which the 

 Lower Cretaceous strata discussed in this article are known to 

 exist, the Lower Cretaceous of California, which contains a 

 totally different fauna, not being now referred to. Important 

 exposures of the Lower Cretaceous strata which are discussed 

 herein are known in the 'Mexican states of Chihuahua and 

 Coahuila, and I have obtained some indirect indication of 

 their presence in the States of JNTuevo Leon and Zacatecas 

 also. They are believed to extend still farther southward in 

 the Republic of Mexico, but I have yet no definite informa- 

 tion of it. 



The greatest known development of these Lower Cretaceous 

 strata, as regards thickness, is reached in northern Mexico, but 

 more abundant collections of the fossil fauna which character- 

 izes them have been obtained within the limits of the State of 

 Texas than elsewhere. This latter fact is probably due not 

 more to the originally greater prevalence of the fauna there than 

 to the greater consolidation which the strata have undergone 

 in their westward extension and the consequent obscuration 

 and inaccessibility of most of the fossils which the rocks there 

 really contain. 



The contrast between the Lower Cretaceous of the South- 

 west and the Upper Cretaceous which overlies it there, and 

 which also prevails in the Atlantic, Gulf and Interior regions, 

 is very great. The former, wherever it has been found, is 

 shown by its fossils to have been an open sea deposit. The 

 strata are either limestones or strongly calcareous rocks, sand- 

 stones and argillaceous shales having never been found to enter 

 largely into their composition ; and, except locally, it has not 

 yet been found practicable to divide them into separate forma- 

 tions upon paleontological grounds. On the contrary the 

 Upper Cretaceous is largely composed of sand and other 

 detrital material, plant remains are frequently found in its 

 strata and, in the great interior region, portions of all the for- 

 mations are coal- bearing. Furthermore, it is divisible into 

 several separate formations which are recognizable over large 

 geographical areas. 



