566 CENTRAL MINERAL REGION OF TEXAS. 



a. The Cavern Subdivision. — A cavernous weathering, soft yellow dolo- 

 mite, sometimes including beds enough to make a section of fifty to one 

 hundred feet. This occurs at the Silurian base north of the Colorado River, 

 in Burnet County, and to some extent farther west, as well as along the 

 southern border area in Mason County. 



b. The Bluff Subdivision. — A bluish, tough, thick-bedded, siliceous dolo- 

 mite, overlying the cavern beds, usually forming vertical bluffs along the 

 streams and in the escarpments where it occurs. Well exposed along the 

 inner northern border of the Silurian eastward, and on Cold Creek, and in 

 parts of the Llano canyons in the southwest. 



2. The Wyo Division. — In some parts of the Silurian area there are 

 craggy cliffs made up of thin shaly beds which overlie the bluff beds, and 

 are themselves overlaid by beds of transition to more pure limestones like 

 the "Burnet marble." There are good exposures of this kind in the country 

 west of James River, in Mason County; also in the region west of Ten Mile 

 Creek, in Mason County, and near the west bank of Little Llano Creek, in 

 Llano County: 



3. The Hoover Division. — In Hoover Valley and eastward and south- 

 eastward in Burnet County, also in a broad belt partly encircling the Central 

 Mineral Region, the Hoover beds are very prominent. They include the 

 largest part of the so-called "lithographic stone" and "Burnet marbles," be- 

 sides a set of fucoidal beds at the base which partake of some of the charac- 

 ters of the preceding division. Near the summit of the Hoover division the 

 strata are rather crystalline, like true marbles, while the inferior members and 

 alternating layers are compact limestones, of a texture resembling the lithog- 

 rapher's material. Fossils are rather abundant in some portions of this 

 terrane. 



SAN SABA SERIES. 



A series of strata allied by the fossils to the Silurian System, but posses- 

 sing some features which make it possible that it belongs in part to the Niag- 

 arian Period, are provisionally put together in a series intended to rank with 

 the Trenton in other States. The rocks are all more or less cherty or sili- 

 ceous, but there is sufficient diversity to separate the members into two 

 divisions. They are confined to the eastern and northern portions of the re- 

 gions under review, being especially prominent in San Saba County south of 

 the San Saba River. 



1. The Hinton Division. — The lower portion of the San Saba series has 

 best expression in sections along the course of Hinton Creek and eastward, in 

 San Saba County; in the mid-course of Cold Creek (east bank), in Llano 

 County; and in parts of the San Saba River bottom west of Camp San Saba, 



