24 BULLETIN" 348, U. S. DEPARTMENT OF AGRICULTURE. 
sedimentary rocks (Nos. 3 and 5), on the other hand, are distinguished 
from those that break down readily under water (Nos. 4 and 6) by a 
much lower percentage of colloidal secondary products (kaolin and 
chlorite) and by a corresponding decrease in cementing properties. 
From what has been said it is evident that basic crystalline and sedi- 
mentary rock powders with high cementing values and abundant 
secondary compounds have a marked tendency to slake, especially 
where the latter products are essentially colloidal in character, 
whereas the slaking property of acid crystalline rocks is influenced 
mainly by the relative abundance of primary quartz and mica and 
bears no very definite relation to secondary mineral composition and 
cementing properties. 
SUMMARY AND CONCLUSION. 
In the preceding pages a microscopic method of rock analysis by 
means of a crossline grating, has been reviewed and the average 
mineral composition and physical properties of all rocks analyzed 
and tested in this office up to January 1, 1914, have been presented 
in tabular form. This material has been classified according to com- 
position and structure and the rock-forming constituents have been 
discussed under the groups of primary and secondary minerals with 
a view to explaining their essential physical characteristics. The 
physical properties of rocks for road making and the method of testing 
this material have also been given and the results obtained used to 
show, first, in a general way, the relation between these properties 
and the mineral composition and structure of rocks; and secondly, to 
point out more specifically the effects of secondary compounds upon 
them. It has been found that igneous and nonfoliated metamorphic 
rocks owing to a preponderance of firmly united silicate minerals 
combined with uniformity in structure, offer a greater resistance to 
abrasion than other rock types, coarse-grained varieties being less 
tough and having inferior indicated wearing properties to those of 
finer grain in which the mineral components are more closely inter- 
grown. Excessive quantities of glass in volcanic lavas and high 
percentages of readily cleavable mica and feldspar in plutonic rocks 
lower their toughness and indicated resistance to wear; on the other 
hand, appreciable quantities of hornblende and augite have an 
opposite effect. Foliated schists and gneisses parting readily along 
planes of schistosity are low in toughness while sandstones and lime- 
stones owe their inferior indicated resistance to wear either to incom- 
plete consolidation or to a preponderance of softer carbonate minerals 
and clay. 
