650 CENTRAL MINERAL REGION OF TEXAS. 



tain nothing which can be properly classed under this head in their present 

 metamorphosed condition. The earliest strata of the kind are of Cambrian 

 time. 



CAMBRIAN. 



Comparatively few of the Cambrian rocks of our area are suitable for build- 

 ing uses, except as rough foundation blocks, and for rubble, "grouting,'' etc. 

 Such calcareous material as may occur in the Lower and Middle Cambrian is 

 chiefly but the imperfect cementing ingredient of more, or less arenaceous 

 rocks, and the colors are very rarely sightly, usually shades of red or yellow. 

 Few of them are capable of withstanding even moderate stresses in structural 

 work. Exception may possibly be made in favor of the buff dolomites near 

 the top of the middle series, which have not yet been actually tested critically. 

 Examples of these last are exposed in the middle part of the canyon of Bluff 

 Creek, Mason County, and to some extent in the cliffs of the Big Sandy Can- 

 yon at the water gap in the Riley Mountains, in Llano County. 



The Upper Cambrian contains black dolomites near its middle portion 

 which seem to possess good qualities as building stones for situations not de- 

 manding select tints. The general character of these layers is well displayed 

 in the phototype illustration of a cliff in Silver Mine Hollow, Burnet County, 

 accompanying this Report. 



These rocks are shaly limestones, or calcareous shales. They are accessible 

 in many places; as on Lion Mountain, Morgan Creek, Beaver Creek, Back- 

 bone Ridge, etc., Burnet County; over a limited area about Cherokee, San 

 Saba County; to some extent in the neighborhood of Camp San Saba, Mc- 

 Culloch County; near the base of some of the ridges lying off eastward from 

 Blowout Postoffice, Blanco County, and in the country west and south of Ma- 

 son City. This material has been successfully used for architectural purposes 

 to a limited degree in small structures, and the prospects are very good for 

 its future usefulness. Houses, stores, mills, and other buildings in Mason, 

 Camp San Saba, and the adjoining country have been built of this rock, and 

 some of them have been standing for many years. 



SILURIAN. 



Aside from the inferior grades of Burnet marble, which may often serve 

 admirably as simple dolomites, there are many other layers in the Silurian 

 System more or less adapted for constructive work. Taken as a whole, these 

 rocks rank as siliceous magnesian limestones, the majority of them being 

 tough and of lighter hues than the earlier dolomites. Usually they weather 

 more evenly and take on less objectionable tints than their predecessors upon 

 exposure to the elements. 



