10 



limestone above probably belongs to the Trenton period, 

 indicated by shells of Orthis, Strophomina, Leptena and 

 crinoids, specimens of which I collected last Spring. 



The lower Potsdam sandstones have the peculiar green 

 sand (glaxiconite), common to the rocks of the period m 

 Burnet and Llano counties. The entire strata of the 

 mountain has a thickness of about one thousand feet and 

 all horizontal, most of the adjacent mountains being 

 granite. 



Sandstones of the Potsdam period abound in Mason 

 county, and extend westward in Menard county, nearly to 

 Menardville. 



There is a fine exposure of these rocks in the western 



Eart of Mason county, on the Llano river, at the foot of 

 lOyal valley. 



Rocks which may and probably do belong to the lower 

 Silurian are upraised at the base of the Organ mountain at 

 Fort Bliss, near the town of El Paso, where there is a fine 

 display of uptilted strata of lower silurian, carboniferous 

 and cretaceous rocks to a nearly perpendicular height of 

 1600 feet above the valley. 



CAKBONIFEROTJS. 



The rocks of this age in Burnet and Llano have been 

 alluded to in former reports, and are noticed elsewhere 

 under the head of coal. 



CEETACEOtrS. 



These rocks have been traced the past season from -a few 

 miles west of Menardville to Fort McKavit, thence north- 

 ward as far as Kickapoo springs, south of Fort Concho , 

 again, from the Twin mountains, about six miles west of 

 Fort Concho, westward to near Barilla springs, beyond 

 Fort Stockton ; thus far with no interruption by other 

 rocks ; thence westward to the Rio Grande. We saw no 

 other sedimentary rocks except cretaceous, and only these 

 at rare intervals, in uptilted strata among or on the sides 

 of the granitic and basaltic mountains of that region. 

 North of the Toyah valley are many limestone mountains 

 which are probably of cretaceous age. 



About four miles north of El Paso, above the Pass of 

 the Rio Grande through the mountains from which the 



