638 CENTRAL MINERAL REGION OF TEXAS. 



this purpose. The expenditure of a few hundred dollars could be made to 

 reveal a large amount of practical information of the very highest importance 

 to architects, builders, and constructing engineers. 



For present purposes a classification by substances is manifestly the most 

 convenient. The granites and marbles are the most important products of 

 this group, but they do not by any means include all the building materials 

 of value. 



A. THE GRANITES. 



In the absence of tests of strength or of convincing practical records of use 

 of any but the now celebrated " Capitol Granite," it is not possible to suggest a 

 better arrangement of the granites than that which was offered by the writer 

 in 1889 (p. 365.) The new material given here will be largely in the nature 

 of a review of the distribution of the several grades of granite. Arrange- 

 ments have been made for the intimate study of these rocks by a specialist in 

 microscopic petrography, and the grouping adopted here is therefore subject 

 to such revision as may be demanded after a more complete knowledge is 

 acquired. Although practical men use other methods of determining values 

 and applications, their conclusions, if trustworthy, will invariably be found to 

 tally with those of scientific investigators, and the more we can learn of the 

 composition and textural arrangement of the mineral components, the better 

 shall we be able to determine the economic value of each particular kind of 

 rock. 



Dr. William C. Day, in Census Bulletin No. 45 on the ''Granite Industry 

 of the United States," issued March 26, 1891, thus aptly remarks: 



Although variations in the nature and proportions of the minerals which constitute the 

 granites have much to do in determining the adaptability of the stone to many purposes, 

 still this fact is not made prominent by granite quarrymen in placing their products on the 

 market. If by actual use a particular granite is found to do well for a certain purpose, it is, in 

 general, correspondingly well received, without inquiry as to its special constitution, which 

 in reality determines its adaptability for such purpose. 



.~~ The age and geologic environment of each granite in our area are important 

 items in determining conditions affecting quality and quantity, but the study 

 of such matters in the field is too complicated for any but skilled engineers. 

 For general purposes, therefore, a simple classification based upon the tooling 

 qualities of the material will be most practical. Quoting from my report for 

 1889: 



When numerous quarries are shipping the different grades, a commercial classification 

 may be adopted which will not agree closely with the one here proposed, but there is a kind 

 of textural relationship which makes this provisional arrangement at least of temporary 

 value. Adopting, then, as a basis of affinity the structural character, using texture, color, 

 etc., as a means of further division when necessary, we get seven fairly distinct classes. 



