PHYSICAL PROPERTIES OF KOAD MATERIALS. 17 



andesite by averaging the analyses of fresh and altered varieties; and 

 in the case of basalt and diabase, by averaging samples containing 

 less than 1 per cent, from 1 to 5 per cent, from 5 to 10 per cent, from 

 10 to 20 per cent, from 20 to 30 per cent, and all above 30 per cent, of 

 secondary minerals. 



It will be observed in the first place that, in contrast to coarser- 

 grained plutonic rocks the toughness and hardness of unaltered vol- 

 canics (Nos. 1, 6, and 12) are as a rule superior to those that have 

 suffered from the effects of weathering. Exceptions will be noted, 

 however, in the case of unaltered vitreous andesites (No. 4), which 

 are low in toughness and indicated wearing properties on account of 

 excessive glass content, and in that of certain highly altered varie- 

 ties of trachyte, andesite, basalt, and diabase (Nos. 3, 5, 10, 11, and 

 17), which owe their relative high toughness and good indicated 

 wearing properties to the development of hard secondary epidotes 

 and zeolitic compounds combined with but little glass. The cement- 

 ing values of volcanic rocks increase up to a certain point quite 

 uniformly with increased proportions of alteration products, reach- 

 ing a maximum in quartz-free rocks (Nos. 5, 11, and 17). Expressed 

 numerically, this increase is 126 for rhyolite and 241 for andesite 

 with equal percentages of secondary minerals (21.6 per cent), while 

 basalt and diabase having about 50 per cent alteration compounds 

 show a maximum increase of 201 and 129, respectively. 



Comparing these results with the foregoing, it is apparent that the 

 cementing property of volcanic rocks is on the whole superior to 

 that of plutonic rocks having equal amounts of secondary minerals, 

 and, furthermore, that this property is here less influenced by pri- 

 mary mineral composition — quartzose rhyolites binding almost as 

 well as basalts (Nos. 1, 2, 6, and 10) — but is effected chiefly by dif- 

 ferences in secondary compounds. This will be made clear by noting 

 the average cementing values for basalt and diabase, which increase 

 quite uniformly with the rate of decomposition where softer colloi- 

 dal compounds, such as kaolin and chlorite prevail (Nos. 6-9 and 

 12-16). Beyond this point the presence of harder, more highly 

 crystalline products of alteration (epidote, zeolite), indicating a 

 less colloidal condition of the other secondary minerals, lowers the 

 rate of increase (Nos. 11 and 17) or causes a falling off in binding 

 properties (No. 10). 



SEDIMENTARY ROCKS. 



In discussing the origin of rocks it has been stated that road ma- 

 terials belonging to this class (limestones and sandstones) are formed 

 chiefly by the cementing together of shell and mineral fragments or 

 by chemical precipitation of amorphous silica (chert). "While the 

 mineral constituents of sandstones and cherts have been determined 



