364 
ROADS. — CANALS. 
compact, will be durable. The most valuable ma- 
terials for roads in this country are the green- 
stone, trap, and hornblende rocks, though even 
these possess different degrees of toughness from 
the same quarry. We may form a very correct es- 
timate of the comparative toughness of different 
stones, by pounding a small piece in an iron mortar. 
By using tough materials, especially in the con- 
struction of Macadamized roads, we not only re- 
duce the expenses of their maintenance, but also the 
annual amount of hinderance caused by the more 
frequent supply of rough, new stones, which tend 
so much to retard the progress of wheel carriages, 
and add to the labour of the horses which draw 
them. 
Canals, — A knowledge of the geographical struc- 
ture of a country through which a canal is to pass, 
is no less necessary than in that of roads. Those 
only who possess this knowledge can judge as to 
the probability of meeting springs of water, of the 
porous or impervious character, as regards water, 
of the rocks to be traversed, and the kinds of rock 
which will be encountered in cutting, and the facil- 
ity with which these may be removed. The geol- 
ogist only can estimate justly the value of the va- 
rious mineral substances which may be advanta- 
geously brought to the canal for the purposes of 
traffic, such as coal, iron, marble, and other min- 
erals. 
Estimates for canals in this country vary, very 
often, wide from their actual cost, from the fact 
that some of the rocks traversed readily absorb 
water, rendering it necessary to incur the expense 
of making the canal-bed water tight. A httle 
knowledge of the nature of rocks in this respect, 
and the geological structure of the country to be 
traversed, would enable the engineer either so to 
locate the canal as to avoid such rocks, or, if that 
is impossible, to make proper allowance in form- 
