654 CENTRAL MINERAL REGION OF TEXAS 



yield good clays, and that a little prospecting will usually decide the question 

 of their existence and character. The Llano River valley from about Llano 

 to its mouth, and the Colorado at least below this point, besides the banks of 

 tributary streams running in granite, afford some of the best material, but 

 the choice clays are not restricted to these areas. Generally speaking, the 

 whole granitic area affords occasional deposits of presumable value, very little 

 of which have been practically tested. These clays are perhaps less plastic 

 than some, but they are usually refractory. Occasional banks of kaolinitic 

 clay occur in restricted areas in the Burnetian districts, but they are not very 

 abundant nor extensive. 



The rocks above the crystalline strata contain but few members which yield 

 clay by disintegration, but certain of the limestones may possess qualities 

 fitting them for use as cements. These have not been so used as yet and there 

 are no data for forming a reliable judgment. In the Cretaceous some beds 

 are well adapted for conversion into Portland cement, or a near ally. This 

 utility is not, however, confined to our border area, for it is characteristic of 

 a very wide belt of Cretaceous exposures eastward and southward. 



G. MATERIALS FOR LIME, MORTARS, ETC. 



Many of the limestones of the Cretaceous border and of the Mason Moun- 

 tain promontory are well adapted for making quicklime, and in many places 

 they are now burned successfully. Some of the Silurian and Cambrian strata 

 might be so employed if necessary, but as a rule these are too siliceous or 

 otherwise unfitted for the purpose. Sands for mortars are extensively abund- 

 ant in much variety, the Cambrian strata affording the best and most exten- 

 sive supply. This terrane surrounds the whole region and is at surface in 

 much of the interior. This distribution renders the sands very readily ac- 

 cessible from all points. The decay of the Trinity Sands (Jurassic or Creta- 

 ceous) along portions of the border gives a "wash" which has often much 

 the same range of usefulness, this terrane being to a large extent the result 

 originally of the degradation of the Cambrian sandstones. 



III. REFRACTORY MATERIALS. 



The rocks which are particularly fire-proof belong almost wholly to the in- 

 terior or pre- Cambrian areas. Soapstone, or steatite, occurs in the Burnetian 

 exposures southwest of Smoothing Iron Mountain, Llano County, also in the 

 northern part of Gillespie County, south and southeast of Enchanted Rock. 

 It is sometimes foliated, but there are some places where it is compact enough 

 to be cut into blocks of sizes suitable for furnace lining. Ordinarily it has 

 but little grit and powders well, so that it is applicable as a fire-proof filling 



