60 A GEOLOGICAL SURVEY OF LANDS 



North of this line the Devil's river limestone rises to form a 

 hilly plateau, and southward it descends under later sediments 

 which form a gradually increasing thickness of cover in that direc- 

 tion. That it extends far out to the south and east under the 

 other formations there can be no doubt. It is not likely to change 

 much in a hundred miles, for it consists of sediments formed in an 

 open sea, whose nearest known shore was far to the north. 



THE DEL RIO CLAY. 



Above the Devil's river limestone there is a clay known as the 

 Del Rio clay. It varies from forty to 150 feet in thickness, and 

 averages probably 100 feet. In a conspicuous hill south of Del 

 Rio, and in the bluff which runs in a northeast-southwest direc- 

 tion to the southeast of this town, several exposures occur, from 

 which the nature of this clay was made out. The upper half con- 

 sists of yellow clay, which is usually calcareous and which contains 

 thin layers of calcareous material and sandstone. In these flags 

 two fossils are invariably present and render this part of the clay 

 always easy of identification, Exogyra arietina and Nodosaria 

 texana. These are equally abundant in other places, from 

 Comstock to Turkey mountain. Some fifty or sixty feet below 

 the top of the clay at Del Rio it contains seams which are more or 

 less impregnated with feruginous material, and the clay itself is 

 of a dark brown or red color, which is due to the presence of hem- 

 atite. The lower half of the formation is usually of a dark green- 

 ish-gray color. 



Mineral Occurrences. 



Gypsum and marcasite were both observed as occasional min- 

 erals in this clay. The latter has usually been changed to hem- 

 atite, which often occurs in pseudomorphs having the cubic forms 

 of the original mineral. This is the case in block "A" and 12 on 

 Devil's river, where some prospectors have mistaken this mineral 

 for galena (sulphide of lead), which has the same crystalline form. 



East of Del Rio hematite occurs as a heavy infiltration in cer- 

 tain layers which are best exposed in the hills about one third of 

 a mile south from the north corner of survey 604 of the G. H. & 

 H. Railroad Company. About ten feet of the clays have been 



