358 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



part, according to G. H. Girty). The Escabrosa succeeds the Martin limestone 

 (Devonian) apparently conformably; it is in turn conformably succeeded by the 

 Naco; and the section ends at the top in a surface of pre-Cretaceous erosion.^^*" 



The Modoc hmestone (Clifton district) is a remnant of the Mississippian series, 

 about 170 feet thick, which rests conformably upon the Morenci formation (Devo- 

 nian?), and is hmited above by a surface of erosion. The Tule Spring limestone is 

 a representative of both the Mississippian and the Pennsylvanian series. 



641b 



H-I 13. TBANS-PECOS TEXAS. 



With reference to the Carboniferous in trans-Pecos Texas Richardson ^^ 

 states : 



The Hueco limestone outcrops in an area of several hundred square miles in trans-Pecos 

 Texas. It underlies the Diablo Plateau, a large area between the El Paso and Van Horn 

 quadrangles, and outcrops in the Sierra Diablo, Finlay, Hueco, and Franklin mountains. The 

 Hueco is a rather homogeneous gray limestone, generally massive, though in places it is thin- 

 bedded. It is comparatively free from chert and differs from the limestones of Silurian and 

 Ordovician age in that it contains little or no magnesia. Although the limestone is prevailingly 

 gray, there are local variations in color from light gray to almost black. 



In the Franklin and Hueco mountains the Hueco limestone immediately overlies the 

 Fusselman limestone, apparently conformably, in spite of the fact of the great hiatus indicated 

 by their ages. But in the Van Horn region a well-developed basal conglomerate averaging 

 approximately 100 feet in thickness and composed of pebbles of aU the pre-Carboniferous 

 formations is present at the base of the limestone, which rests with marked irregularity on 

 the underlying formation. The Hueco limestone generally is overlain by Pleistocene d6bris, 

 but in a few areas, notably in the Finlay mountains and also 8 miles northwest of Van Horn, 

 it is directly overlain by Cretaceous strata. The total thickness of the Hueco limestone has not 

 yet been determined, but it is more than 3,000 feet. 



The Hueco limestone carries an abundant fauna of Pennsylvanian age. * * * Accord- 

 ing to Dr. Girty, this fauna with some modifications is similar to that found over much of the 

 Cordilleran region, and the Hueco limestone is tentatively correlated with the Aubrey formation 

 and the Weber quartzite." 



In the Van Horn quadrangle and north of it the bolson plain known as Salt Flat, which 

 is occupied by an unknown depth of unconsoUdated Quaternary deposits, hes between the 

 Sierra Diablo on the west and the Guadalupe-Delaware Mountains on the east and completely 

 conceals the relations of the rocks in the two mountains. As has been stated, the Sierra Diablo 

 is made up of the Hueco limestone, the stratigraphic top of which has not been observed, and 

 the Guadalupe-Delaware Mountains are composed of Paleozoic strata, younger than the Penn- 

 sylvanian, which contain a fauna not elsewhere known in North America, that Girty has named 

 Guadalupian. This fauna, which has Permian affiliations, is described by Dr. Girty in a paper 

 now in press,'' and it is intended here only to outline the stratigraphy of these rocks, which 

 complete the long Paleozoic sequence of trans-Pecos Texas. 



The Delaware Mountain formation includes a varying mass of sandstone and limestone 

 having a maximum tliickness of at least 2,300 feet, but the base of the formation is not exposed 

 in Texas and has not been determined. In the northern part of the Guadalupe-Delaware 

 Mountain uplift, the formation is prevailingly sandy and contains only thin beds and lenses of 

 limestone. Southward the sandstone decreases and the limestone increases in amount until, 

 in the southern part of the main Delaware Mountains, the formation consists of gray Hmestone 



o Proc. Washington Acad. Sci., vol. 7, 1905, p. 14. 



6 Since published as Prof. Paper IT. S. Geol. Survey No. 58, 1908.— B. W. 



