12 BULLETIN 348, U. S. DEPARTMENT OF AGRICULTURE. 
have been separated into plutonic (Nos. 1-7) and volcanic varieties 
(8-14). The former, embracing rocks of granitic character, are in 
general less tough and have indicated wearing properties inferior to 
their finer-grained, volcanic equivalents. This may be ascribed 
directly to their coarser grain and to the marked effect of feldspathic 
cleavage (seep. 7). It will be observed, furthermore, that among the 
plutonic rocks those varieties unusually rich in mica (No. 2) and sec- 
ondary serpentine (No. 7) have exceptionally low indicated wearing 
properties, whereas samples containing hornblende and augite in 
appreciable quantities are relatively tougher and in general offer 
greater resistance to abrasion (Nos. 3-6). In regard to the cementing 
value of plutonic rocks it will be seen that the quartzose varieties 
(Nos. 1, 2, 3, and 5) are lower than types in which quartz is absent, 
or present only in subordinate quantities (Nos. 4, 6, and 7). 
The high toughness and excellent indicated wearing properties of 
volcanic rocks is in accordance with their fine grain and close inter- 
locking mineral structure. The results of the tests indicate that 
siliceous varieties (No. 8) are on the whole harder though less tough 
than the more basic rocks to which trap belongs (Nos. 9-14). It 
will be observed, also, that samples rich in glass (Nos. 10, 11, and 12) 
are lower in hardness and. toughness than their corresponding wholly 
crystalline varieties (Nos. 13 and 14). The latter are, furthermore, 
characterized in general by a peculiar intergrowth of plagioclase and 
augite (ophitic structure) which indicates greatly increased durability. 
(PI. III). The average cementing values of fresh trap varieties (Nos. 
11 and 13) are much below the corresponding varieties that have 
undergone alteration (Nos. 12 and 14), although superior to the latter 
in other respects. 
Considering briefly the group of sedimentary rocks (Nos. 15-20) 
it will be noted that they are characterized in general by a lower 
toughness and hardness and higher per cent of wear, owing to the 
presence of soft, readily cleavable calcite and dolomite, as well as to 
a more open texture resulting in the case of the sandstones from 
incomplete cementation. (See p. 4.) 
Metamorphic rocks have been divided into foliated (Nos. 21-28) and 
nonfoliated or massive varieties (Nos. 29-35). The former, owing 
to the peculiar parallel arrangement of their mineral constituents or 
schistose structure (PI. VI) are comparatively low in toughness and 
indicated wearing properties except when hornblende is present in 
appreciable quantities (Nos. 27 and 28). Nonfoliated varieties, on 
the other hand, representing the direct result of compact crystalliza- 
tion without shearing of quartz, and hard silicates such as horn- 
blende, augite, epidote, and garnet, are exceptionally hard and tough 
but usually deficient in cementing properties (Nos. 30-34). 
