PHYSICAL PROPERTIES OF ROAD 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 
