BUILDING STONES. 639 



* * * Only in a general way may this be regarded as a scientific grouping, but in pro- 

 posing it an attempt has been made to get as near to the order of formation, or age, as can 

 well be done, etc. 



The seven classes of granite referred to are as follows: 



1. G-neissic Granites (largely Burnetian). 



2. Compressed Granites (intrusives). 



3. Block Granites. 



4. Friable Granites (perhaps more commonly Texian). 



5. Mixed Granites (of Silurian type). 



6. Dimension Granites (largely Carboniferous). 



7. Fissile Granites (Cretaceous type). 



The Economic Map is published upon too small a scale to admit of making 

 these nice distinctions. 



1. THE GNEISSIC GKANITES. 



It is not strictly true to say that all the gneissoid granites are members of 

 the Burnetian System, and yet the gneisses and granites of that age are almost 

 invariably of this class unless they bear evidence of intrusive origin. We 

 shall, therefore, go very little amiss in denning the areas of outcrop as prac- 

 tically coincident with those of the Burnetian System. Perhaps some Fer- 

 nandian outcrops may include thin seams of material which would properly 

 come under this same head, but none of those which I have determined to be 

 of that system are liable to possess any important economic value. Within 

 the range allowed for this class of granites there is considerable variation in 

 texture and color as well as in the essential elements which affect the quality 

 of building stones, such as strength, density, porosity, and tendency to lami- 

 nation or to irregular weathering. Many of them are too brittle, too thinly 

 bedded, or too uneven in texture to afford blocks or slabs which can be ap- 

 plied to any use excepting where ballast or rubble is required. Eventually 

 even this material may meet its destiny, for it is very probable that it could 

 be economically and advantageously employed as the basis of a special con- 

 crete or artificial stone, and no tests are needed to prove its admirable quali- 

 ties as road material. Some of the best natural highways in the world are 

 those in the great Burnetian plain of Llano County, over much of which area 

 the gneissoid granites form the soil. What is locally known as " rotten gran- 

 ite" consists merely of the disintegrated gneiss of the region, which is often 

 thus weathered to the depth of fifteen to twenty feet from the surface. Such 

 material could be cheaply mined, and under favorable circumstances it would 

 bear transportation for long distances. I believe it is destined one day to go 

 far toward solving the now serious road problems of the limestone regions of 

 Central Texas. Although this " rotten granite" is itself a proof of the in- 



