


ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 343 
the direction of the planes of cleavage. Should the latter 
be traversed at right angles, the rock on both sides of the 
cutting will stand with vertical faces. When an excavation, 
however, is carried in the same direction as the strike of 
the cleavage, the rock will necessarily be more stable on one 
side than the other. In a word, the superinduced cleavage- 
structure plays the part of lamination and bedding in the 
case of sedimentary strata. 
It is not necessary to add more than a word as to 
excavations in incoherent and non-consolidated rocks. Rocks 
of this kind will not stand with a vertical face, except in 
rainless or all but rainless regions. Under ordinary conditions, 
therefore, the engineer, in excavating soft, incoherent masses 
or beds, has to consider the slopes he must give to the sides 
of the cutting, for the angle of repose varies directly as the 
nature of the materials. Even in such cases it will usually be 
found that geological structure has its influence. Should the 
incoherent beds have a dip and be traversed at right angles to 
their inclination, they will almost invariably stand better on 
one side of a cutting than the other. Any water that may 
permeate the beds will tend to come out rather on the “weak” 
than on the “strong” side; slipping will be apt to take place 
from time to time on the former, but not so readily on the 
latter. 
Tunnels.—lIf it be unwise on the part of an engineer to 
undertake important excavations or open cuttings without 
having previously examined the geological structure of the 
ground to be traversed, he would be deserving of censure if, 
before driving an important tunnel, he did not first endeavour 
to ascertain every fact connected with the rocks and their 
arrangement. In the case of horizontally bedded strata, not 
much difficulty would arise; the tunnel would be driven 
practically along the planes of bedding, and the character of 
the rock at the two ends of the proposed tunnel could be 
readily ascertained, and thus a reliable estimate of the cost of 
the work could be formed. But if the strata were not 
horizontal, then even the most careful examination of the 
rocks at either end of the proposed tunnel might deceive the 
engineer who had neglected to ascertain the geological struc- 
ture. The annexed diagram (Fig. 125) represents the 
