BUILDING MATERIALS. 367 



feldspar crystals. Some of the block granities are less ferruginous and 

 paler in color than the classes previously described, but when there is an 

 abundance of siliceous cement this loss of iron does not appear to weaken 

 the combination. 



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

 original crystals 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 cer- 

 tain extent, they would probably succumb to excessive temperatures, and they 

 are illy fitted to resist sudden changes of temperature. As a rule they are 

 not ornamental. 



D. THE FRIABLE GRANITES. 



Probably many of the other granites will decay and crumble under the 

 influences of stratigraphic agents, and much attention ought to be paid to 

 the surface exposures and the selection of the best qualities in every case. 

 Some technical knowledge and practical experience is necessary to enable 

 one to choose the material understandingly. In places, as much as twenty - 

 five to thirty feet of rotten granite is now covering hard granites of the same 

 class from simple decay in situ. The friable granites here included, however, 

 belong to another category. They are perhaps as well described by the 

 title "sandy granites." as they are fine-grained, granular, not very coherent, 

 and commonly quartzose. These occur abundantly in our region, especially 

 in the border areas, representing chiefly a Pre-Cambrian uplift, and they 

 sometimes grade off into sandstones. Another older set, which may be in- 

 cluded with these, comprises the sandy gneisses of Lockhart Mountain and 

 other districts. None of these have any value in construction, although it 

 may be possible to utilize a portion in some way connected therewith. Their 

 best application will be for road making. 



E. THE MIXED GRANITES. 



The Post- Silurian uplift, as previously announced, brought up a magma 

 which is now exposed in Honey Creek, Llano County, and in Honey Creek, 

 Mason County. This has been referred to as the bearer of schist inclusions. 



