24 BULLETIN" 348, U. S. DEPARTMENT OP 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 cf 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. 



