PALEOZOIC GROUP. 565 



2. The Potsdam Flags. — Shales and thin-bedded limestones exposed in 

 good sections along Beaver Creek, Burnet County, and near Camp San Saba, 

 McCulloch County, but also visible in many other Potsdam sections. (See 

 photo-engraving, this Report.) 



3. The Potsdam Limestone. — A peculiar shingly limestone conglomer- 

 ate, overlaid by limestone (fossiliferous) of a greenish color in large measure, 

 characteristic of the Cambrian sections in the northwest, but appearing in 

 other places. 



5. SILURIAN SYSTEM. 



As a whole the Silurian* of Central Texas is a system of siliceous dolo- 

 mites and chert. There is some variety in the beds, but they embrace the 

 so-called "marbles" and their associates, as distinguished from the "moun- 

 tain limestone" of the local vernacular. The topography of the Silurian re- 

 gion is uneven, the creeks running much in canyons, and agriculture is 

 chiefly confined to the bottom lands along the streams. The soil is usually 

 thin and strewn with rocks, making wagon travel across the country very 

 difficult. Almost the whole of the region where these strata outcrop is used 

 for grazing, and there are no important minerals besides the veins of calcite 

 and hydrated iron ores of limited extent. The Silurian beds lie upon the 

 outer borders of the Central Mineral Region, being well developed in parts 

 of Burnet, San Saba, McCulloch, Mason, Blanco, and Gillespie counties, and 

 to a slight extent in Kimble County. 



The best stratigraphic classification which we have yet been able to adopt 

 is that announced in the First Annual Report of the Survey, f making two 

 series, as below : 



LEON SERIES. 



The lower members of the Silurian System are shaly at base, becoming 

 gradually less impure and even crystalline at the summit of the Leon series. 

 They are best exposed in Burnet, McCulloch, and Mason counties. Three 

 divisions have been established, which are not, however, of sufficient import- 

 ance economically to be separated in practical explorations, although the 

 "Burnet marble" is the chief constituent of one of them. 



1. The Beaver Division. — The lowest members of the Leon series are 

 magnesian limestones, of yellow and bluish hues, having characteristics suffi- 

 ciently distinct to entitle them to be classed separate subdivisions. 



*Following Dr. J. D. Dana, this term is used for the Lower Silurian, as formerly under- 

 stood. For the old term Upper Silurian the name Niagarian is substituted here, as in my 

 report for 1889. T. B. C. 



+ Page 306. 



4:4: — geol. 



