252 Huntington — Road-Making Materials. 
the preceding. The other granite was quite coarse grained 
and contained a small amount of hornblend with a large pro¬ 
portion of quartz. The gravel used as cement was taken just 
as the pebbles happened to come and represents the average 
gravel of Southern Wisconsin. The sand was taken from the' 
drift and had a little clay intermixed. The limestone was of 
the Trenton variety, the buff being from the upper layer and of 
average hardness. The blue limestone was from a shaley layer 
and is the most compact and durable part of the Trenton lime¬ 
stone. It is, however, decidedly softer than the average peb¬ 
bles of the drift. 
If the materials used as cement in numbers ten to fourteen,, 
viz., gravel with a value of 43.7, buff limestone with a value of 
43.6, medium granite 43.6, sand 36.4, and coarse granite 27.3, 
could be placed on the road at equal cost, it would evidently be 
folly to use either very coarse granite or sand as far as bind¬ 
ing is concerned. In many places where coarse granite can be 
secured cheaply it may, of course, be profitably used as the 
major part of the macadam if some first-class binding material 
is added. The other three materials, gravel, quite fine-grained 
granite, and rather soft limestone show the same cementing 
value of 43. The limestone is so soft that it can never be used 
profitably. Granite is usually harder than gravel, and also, for 
use as the main material of a road, it has another decided ad¬ 
vantage. The results of a few experiments in which crushed 
stone was used, and not crushed gravel, bring out this disad¬ 
vantage of gravel. In ordinary crushed gravel only about two- 
thirds of the surface consists of fresh fractures, and these alone 
cement readily. The other third is rounded and worn and not 
only does not cement readily, but also by its roundness tends to 
be thrown out and to make the whole mass of road material less 
firm and solid. This is an evil which cannot be avoided if gravel 
is used, but it may be lessened by rejecting a greater proportion 
of the smaller stones. It would be worth while to transport 
good granite at quite an expense by rail even if good gravel were 
close at hand. 
Numbers 8 and 9, which are mixtures of medium granite and 
limestone, and of coarse granite and limestone, show about 
