there are many large dikes of basaltic rocks, some of 

 which extend across the Rio Grande into Mexico. 



The Chinati mountains, in the southwest part of Presi- 

 dio county, are chiefly igneous rocks, granites, dolerites, 

 etc. 



On the road from Presidio to Fort Davis, about forty 

 miles south of the latter, are low hills of dolerite and 

 basaltic rocks. 



All the igneous rocks that we saw north of the Pecos 

 are either of upper cretaceous or tertiary age, shown by 

 the uptilted strata of the cretaceous rocks amid and bor- 

 dering the mountains. The Cerro Hueco mountains may be 

 an exception. I saw no tertiary fossils beyond the Pecos. 



OTHER BASALTIC BOCKS. 



About four miles east of Port Clark, in Kinney county, 

 on the San Antonio road, are dolerite and basaltic rocks, 

 extending fifteen to twenty rods along the road, and cover- 

 ing much of the hill over which the road passes. 



Again, about two miles west of the Nueces, on the same 

 road, there is a similar outcrop. 



On the west bank of the Frio at the ford, there is a large 

 mass of basaltic rocks about fifty feet high, extending 

 several hundred feet up the river. On the eastward from 

 the Frio, I saw dolerite rocks in the road and near it quite 

 often. At one place, about four miles eastward from the 

 river, there is a mass of dolerite covering several acres, 

 surrounded by cretaceous limestones. 



Again, Pilot Knob, about ten miles southwest of Austin, 

 is dolerite, and here these rocks extend three or more miles, 

 also surrounded by cretaceous limestones. 



There are probably other localities of these rocks along 

 the southern borders of the cretaceous region of the 

 State.' V 



LOWER SILURIAN. 



Xhe rocks of the lower silurian in Burnet and Llano 

 counties have been noticed in previous reports. To define 

 them correctly, a detailed survey is needed. 



In the northwest part of Llano county, • about two miles 

 southwest of Smoothing Iron mountain, is a mountain' 

 with a sandstone base and limestone top, all of lower 

 Silurian ; the sandstone below of the Potsdam period, the 



