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 (see p. 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). 



