372 CENTRAL MINERAL REGION OF TEXAS. 



can be made applicable. It will be necessary to make special search for them. 

 The area of most promise is between the Riley Mountains and the west line 

 of Llano County, or a little further west in Mason County. The red color is 

 against some which might otherwise be serviceable, and others are too fragile, 

 owing to the ready decomposition of a feldspathic ingredient. The sand- 

 stones proper are all Cambrian. Some of these upon the outer edges of the 

 district may be adapted for building uses, but they are not all of such quality 

 as to make them desirable. By selection it may be possible to get some very 

 uniform and pleasing varieties of red sandstone, but the difficulty will be to 

 find places where metamorphism has sufficiently hardened the rock without 

 at the same time cracking it too much for service. A notable exception is 

 the yellow indurated sandstone of the Lower Cambrian. This, as exposed 

 upon Packsaddle Mountain, House Mountain, Smoothing Iron Mountain, and 

 elsewhere, is a massive tough rock, semi-quartzitic, with siliceous cement, 

 occurring in layers ten feet and more in thickness. Undoubtedly useful appli- 

 cations of such material will not be lacking, and it is often favorably exposed 

 for working. 



The white sandstones, like the red, are usually too fragile for purposes of 

 construction except for use in mortar and plaster. 



5. SLATES, PORPHYRIES, ETC. 



There is some prospect that certain of the fissile strata of the Archaean and 

 Algonkian belts may become valuable as roofing slates, although the surface 

 exposures are often so much altered that determinations of quality are not 

 always conclusive. The area in which the Burnetan, Fernandan, and Texan 

 rocks are now uncovered will be the proper places in which to seek these 

 outcrops, but the later uplifts have occasionally brought them to view in a 

 more or less contorted condition. To test material of this class, it should be 

 soaked for a day or two in water, subjected to a drying heat, and then sud- 

 denly plunged into cold water. If the absorption be slight and the materials 

 do not materially change by flaking, crumbling, or softening, the rock is 

 liable to be serviceable, although one familiar with such tests should be called 

 upon to decide finally. The porphyries do not impress one with being of 

 much economic importance except in a few instances. There is a tough, dark- 

 green to black hornblendic rock crossing the upper valley of Cold Creek in 

 the northern portion of Llano County, which may perhaps be utilized in some 

 way. Probably it might make good paving material, and possibly it could 

 be successfully used for ornamental use in building. 



There are also some red porphyries which a vigorous development of the 

 interior may eventually make valuable in the arts; but with the foregoing 



