CLAYS, CEMENTS, ETC. 653 



monly break with the slaty fracture. The Texian strata contain less crystal- 

 line members, some of which are more argillaceous. The dynamic results 

 within the whole pre-Cambrian area are not in the nature of lateral thrusts, 

 except in such a limited degree as has been requisite for contorting the layers 

 without producing the slaty cleavage. 



Some of the more friable schists may probably be utilized some day as the 

 basis of substantial and attractive concretes. Their variety, accessibility, and 

 ease of manipulation leave little to seek for this purpose. 



Serpentines and other special products are not exposed in very extensive 

 tracts, but there are some very good deposits in regions of much disturbaDce, 

 as in the vicinity of Enchanted Rock, on Kuehne Creek, and elsewhere, Gil- 

 lespie and Llano counties; and in the region of the King Mountains and Bode 

 Peak, Llano and Mason counties. It is not probable that these will furnish 

 vast supplies, although specimens of much beauty may be mined occasionally, 

 and the general product is tough and fairly uniform. 



F. CLAYS, CEMENTS, ETC. 



Reference will be made to certain refractory materials in another place. 

 The clays of the Central Mineral Region which are adapted for brick and 

 tile making are more abundant than would be inferred from casual inspec- 

 tion. Notwithstanding the general appearance of pure sand which large 

 areas of the soil exhibit, the amount of feldspathic ingredients in the "gran- 

 ite wash" is very considerable; the erosion has been so thorough and the as- 

 sorting action of water so complete that many accumulations of silted clay 

 occur in all parts of the district. There is as much variety as might be an- 

 ticipated in the characters which furnish the basis for grading such deposits. 

 Coarse, fine; unctuous, friable, pulverulent; sandy, limy, mucky; red, blue, 

 white, and other hues; all may be found in quantity and in many different 

 situations. Owing to the numerous changes of condition which these accu- 

 mulations have undergone since the degradation of the original rocks, it is im- 

 possible to draw sharp lines of demarkation between the local patches, nor 

 can one safely generalize or lay out the metes and bounds of individual traclis. 

 Often the best material is covered by later soil of a different character, and 

 the conditions are ordinarily so purely local that a test of one spot can not be 

 used as a guide in the search for others. Our work in these surface deposits 

 has necessarily been incidental rather than specific. But enough has been 

 learned to show that there are comparatively few reaches along the flood 

 plains of the larger streams, back from the more recent overflows, in which 

 clays of some kind do not abound. Starting with this assumption, let it be 

 borne in mind that the areas in which the silt is local in regions of shales, and 

 often the reverse in limestone and sandstone districts, is the most liable to 



