14 BULLETIN" 348, TJ. S, DEPARTMENT OF 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 plu tonic 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) - 1 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 f amilies containing approximately like propor- 
tions of secondary minerals, obtained in the case of rhyolite and 
i Office of Public Roads Bui. 37, p. 18. 
