642 CENTRAL MINERAL REGION OF TEXAS. 



As limited above, the thickness of these rocks varies from a single layer of original crys- 

 tals to an aggregate of one foot in all. They occur all over the district, but are perhaps most 

 common in the mid-belts of Llano County. In the absence of actual tests it is not possible 

 to state their applicability in the building art, but it is not probable that they will usually 

 withstand great pressure, unless understandingly applied in masonry structures specially 

 planned to distribute the stresses unequally in definite lines. Care should be taken to study 

 the lamination planes and the position in the quarry, and to lay them in equivalent positions 

 in walls. Some of them may be adapted for use in pavements and in buried foundations. 

 Although refractory to a certain extent, they would probably succumb to excessive temper- 

 atures, and they are illy fitted 10 resist sudden changes of temperature. As a rule they are 

 not ornamental. 



Some of the Burnetian mounds weather in such a way as to produce a 

 comparatively thin "lift," as quarrymen call it, and this portion affords fair 

 examples of block granite in some places, as at Flat Rock, near Lone Grove, 

 and in certain of the hills of the Cat Mountains. West of Cold Creek, in 

 Llano County, near the road to Field Creek, in the neighborhood of House 

 Mountain, and in the region northwest of Castell, there are examples of the 

 class often varying considerably in strength and durability in the same ex- 

 posure At the crossing of San Fernando Creek by the Valley Spring and 

 Mason road, north of Rough Mountain, there is some granite of the "block" 

 type, but its exposures do not give promise of much endurance under pres- 

 sure. Some of the material south of Katemcy, and perhaps other masses in 

 Honey Creek, both in Mason County, may prove serviceable. 



From the base of Enchanted Rock samples of tougher quality have been 

 taken; and generally speaking the more recent granites seem to afford the 

 best specimens, probably because they have been less altered by the ravages 

 of time. 



A little granite of this type occurs also in the Beaver Creek outcrops in 

 Burnet County. The quality there is good, but the color is not the most at- 

 tractive. 



THE FRIABLE GRANITES. 



Over wide areas as much as twenty to thirty feet of "rotten granite" now 

 forms the surface -cap of material of its own class, being the result of decay 

 in situ. As before remarked under the head of the Gneissic Granites, this is 

 very largely of Burnetian age, and the process of rusting has been going on 

 for untold ages. The newer granites are but rarely thus affected, and never 

 to such a depth. But the friable granites here grouped are not of this nature. 

 They are rather what may be called "sandy granites," and are always fine- 

 grained, granular, but loosely aggregated, ordinarily quartzose. Tnese are 

 of abundant occurrence in the Central Mineral Region, for the most part be- 



