100 EXPLORING EXPEDITION FKOM SANTA Ffi 



at the eastern base of the Sierra Abajo. Within the last few weeks we have been on 

 three sides of this sierra, and have learned its structure quite definitely. It is a 

 mountain group of no great elevation, its highest point rising some 2,000 feet above 

 the Sage-plain, or perhaps 9,000 feet above the sea. It is composed of several distinct 

 ranges, of which the most westerly one is quite detached from the others. All these 

 ranges, of which there are apparently four, have a trend of about 25 east of north, 

 but being arranged somewhat en echelon, the most westerly range reaching farthest 

 north, the principal axis of the group has a northwest and southeast direction. The 

 sierra is composed geologically of an erupted nucleus, mainly a gray or bluish-white 

 trachyte, sometimes becoming a porphyry, surrounded by the upheaved, partially 

 eroded, sedimentary rocks. The Lower Cretaceous sandstones and Middle Cretaceous 

 shales are cut and exposed in all the ravines leading down from it, while nearly the 

 entire thickness of the Cretaceous series is shown in spurs which, in some localities, 

 project from its sides ; apparently the remnants of a plateau corresponding to, and 

 once connected with, the Mesa Verde. Whether the Paleozoic rocks are anywhere 

 exposed upon the flanks of the Siei'ra Abajo I cannot certainly say, though we dis- 

 covered no traces of them. It is, however, probable that they will be found in some 

 of the deeper ravines, where, as in most of these isolated mountains composed mainly 

 of erupted material, they are doubtless but little disturbed, but are buried beneath the 

 ejected matter which has been thrown up through them. 



The relations of the Cretaceous rocks to the igneous nucleus of the Sierra Abajo 

 are very peculiar, for, although we did not make the entire circuit of the mountain 

 mass, and I can, therefore, not speak definitely in regard to the western side. As far 

 as our observations extended we found the sedimentary strata rising on to the trachyte 

 core, as though it had been pushed up through them. 



