564 CENTRAL MINERAL REGION OF TEXAS. 



4. CAMBRIAN SYSTEM. 



The Cambrian rocks include sandstones, shales, and limestones, usually not 

 very thick-bedded except in the case of some of the tough conglomerates 

 near the base of the system. Red, white, and green colors are largely charac- 

 teristic and false bedding is very common. There are few compact layers 

 and many of them are friable. They are well represented in the western part 

 of Burnet County, and in parts of Llano County, but more extensively in Ma- 

 son County, the areas of exposure extending also into Blanco, Gillespie, 

 Kimble, McCulloch, and San Saba counties. Valuable sands and important 

 deposits of iron ore (hematite) occur in special localities described elsewhere 

 in this Report. Three series have been provisionally announced by the 

 writer, but it is not an easy matter for one unacquainted with geology to dis- 

 tinguish the strata of the different sets. 



HICKORY SERIES (LOWER CAMBRIAN). 



The lowest Cambrian series is not present in all sections, but in places in 

 Burnet, Llano, and Mason counties there occurs a peculiarly dense yellow to 

 white sandstone of non-homogeneous texture, containing grains and small 

 rounded pebbles of white quartz with similar inclusions of darker hue. This 

 occurs in beds of great thickness, and sometimes mounds of it are left with a 

 glassy exterior. Such are the Castle and similar monumental elevations in 

 Mason County, on Little Bluff Creek, also near a branch of Katemcy Creek, 

 and at other points. This sandstone forms the base of the sandstone capping 

 of Packsaddle Mountain, and it is well displayed on House Mountain, Smooth- 

 ing Iron Mountain, and elsewhere. In some sections it is underlaid by a 

 coarse conglomerate carrying larger quartz fragments. 



RILEY SERIES (MIDDLE CAMBRIAN). 



This is a geologic division not yet securely established, although the writer 

 believes it is founded upon srood stratigraphic evidence. The rocks included 

 are sandy shales, calcareous sandstones, and chocolate sandy limestones, such 

 as abound in portions of the Riley Mountains, in Llano County, and farther 

 west in the southern part of Mason County. 



KATEMCY (POTSDAM) SERIES (UPPER CAMBRIAN). 



There are three well marked divisions in the highest set of Cambrian 

 Rocks, as below: 



1. The Potsdam Sandstone. — Red and white and green fine-grained 

 sandstones, with some coarser layers, especially well developed, as an inner 

 fringe along the northern border of the Central Mineral Region, but not re- 

 stricted to that area. 



