14 BULLETIN" 348, U. S. DEPARTMENT OP AGRICULTURE. 



seated processes of alteration. The effect of these secondary com- 

 pounds is best demonstrated by arranging the various members of 

 each rock class, so far as possible, into groups having approximately 

 the same average proportions of alteration products. 



IGNEOUS ROCKS. 



In view of the great structural differences shown in rocks of this 

 class it will be found of advantage to discuss separately the coarse- 

 grained plutonic and fine-grained volcanic varieties. In Table 4 

 the average mineral composition and physical properties of plutonic 

 rocks are given, arranged so far as possible in family groups having 

 approximately like percentages of secondary constituents. These 

 groups indicate an average of all samples having less than 5 per cent, 

 from 5 to 10 per cent, and more than 10 per cent secondary minerals. 



It will be observed in general that for a given family the toughness 

 and hardness of these rocks tend to increase and the per cent of wear 

 tends to decrease with additional amounts of secondary minerals, 

 excepting in the case of very soft, highly kaolinized gabbro and serpen- 

 tinized peridotite (Nos. 11 and 12). This improvement is less notice- 

 able in granitic rocks (Nos. 1, 2, and 3) than in more basic varieties 

 (Nos. 4-10) and may be attributed to the partial replacement of 

 plagioclase feldspar by hard granular epidote (Nos. 3 and 5), accom- 

 panied in some cases by zeolites (No. 8) or actinolitic hornblende 

 (No. 10) . x The influence of secondary constituents is more apparent 

 in regard to cementing value than to other properties. Thus it will 

 be noted that altered varieties have a higher binding value than 

 fresher samples and that the increase is roughly proportional to that 

 of the secondary minerals. This increase, moreover, varies greatly in 

 different rock types, reaching a maximum of 9 in the case of granites 

 (Nos. 1-3), 31 in diorites (Nos. 6-8), 87 in gabbros (Nos. 9-11), and 

 92 in syenites (Nos. 4 and 5). It will be observed, furthermore, 

 that the cementing value of siliceous varieties are in inverse relation 

 to their quartz content. That is to say, granites, diorites, and sye- 

 nites with averages of 28, 8, and 4 per cent quartz, respectively, have 

 corresponding cementing values of 20, 34, and 90, indicating that the 

 binding properties of coarse-grained rocks may be largely influenced 

 by the physical character of the primary mineral components. 



The effect of secondary minerals on the physical properties is more 

 clearly demonstrated in the following group of volcanic rocks. This 

 group embraces all fine-grained varieties of igneous rocks that, as 

 shown in Table 5, are especially adapted to road building. The rocks 

 have been arranged in families containing approximately like propor- 

 tions of secondary minerals, obtained in the case of rhyolite and 



i Office of Public Roads Bui. 37, p. 18. 



