CAMBRIAN AND LOWEE ORDOVICIAN. 



75 



The beds are commonly from 1 to 2 feet in thickness. Their dominant color as seen in 

 large exposures is dark greenish yellow, whereas the prevaiUng tint of the overlying Martin 

 limestone is dark gray, and of the still higher Escabrosa and Naco limestones white or hght gray. 



In the typical Mount Martin section the Bolsa quartzite is immediately overlain by about 

 40 feet of thin-bedded, very cherty limestones, which break up, on weathering, into thin rusty 

 plates. Above this occur a few beds of gray limestone up to 2 feet in thickness, alternating 

 with fissile yellowish calcareous shales, and with laminated cherty beds, such as have just been 

 described. The upper 100 feet of the formation is made up of rather soft, sandy, thin-bedded 

 gray limestone, with one bed of harder gray limestone 6 feet in thickness about 40 feet from the 

 top. The upper limit of the Abrigo formation is defined in the Mount Martin section by a bed 

 of pure white quartzite about 8 feet in thickness. 



This quartzite is a persistent stratum and is always found immediately underlying the 

 Martin limestone, which carries Devonian fossils. Its thickness, however, is variable and it 

 sometimes grades downward into the upper sandy limestones of the Abrigo formation. It 

 apparently records the consummation of an increasing supply of sandy sediments during the 

 later phases of the deposition of the Abrigo Hmestone and contrasts with the more purely calca- 

 reous beds of the overlying Devonian formation. * * * 



The fossils collected from the Abrigo limestone were submitted to Dr. Charles D. Walcott, 

 who kindly examined them and reported that they are middle Cambrian, indicating a fauna 

 closely resembling that of the middle Cambrian of Texas. 



H 13. TRANS-PECOS TEXAS. 



With reference to the Cambrian and Lower Ordovician in the district about 

 El Paso and to the southeast, Richardson"*'^ states : 



The formations which are the subject of this paper outcrop in the Franklin and Hueco 

 mountains in the El Paso quadrangle, and in the Sierra Diablo, Delaware Mountains, and 

 associated groups of hills in the Van Horn quadrangle. These two quadrangles, which have 

 recently been mapped by the United States Geological Survey, are situated about 60 miles apart 

 and they include practically all of the known occurrences of lower Paleozoic rocks in trans- 

 Pecos Texas. The following table summarizes the Paleozoic formations in the El Paso and Van 

 Horn quadrangles: 



Paleozoic formations in the El Paso and Van Horn quadrangles, Texas. 



