

344 STRUCTURAL AND FIELD GEOLOGY 
geological structure of a hill which it is proposed to tunnel 
along the line a 4@, 
borings put down over the surface can give no indication of 
the nature of the rocks which the tunnel is likely to pierce. 
SS 

~ -”- 
WSsu Lee” 
FIG. 125.—TUNNEL THROUGH SYNCLINAL STRATA. 
It is obvious that shallow pits and 
An engineer, finding that the rock 
over the top of the hill and at the two 
extremities of the proposed tunnel, 
were all of a reliable kind, might 
probably conclude that the whole hill 
was composed of like materials. If 
the rock happened to be of a self- 
supporting nature—one that required 
little or no expensive building—he 
would frame his estimates of cost 
accordingly. A knowledge of the 
actual structure, however, would have 
shown him that the self-supporting 
stratum could continue but a short 
distance on the level of the proposed 
tunnel, and would then be succeeded 
by friable shales requiring support all 
the way to near the middle of the hill, 
where highly porous and water-logged 
sandstones might be expected to add 
still further to the difficulties and cost 
of the undertaking. The history of 
engineering operations in this and 
other countries is full of warnings as 
to the danger of driving tunnels with- 
out having first determined the geo- 
logical structure of the ground. Not 
infrequently, this requisite knowledge 
might have enabled the engineer to 
avoid difficulties and greatly increased 
cost by some slight deviation of the line 
of his tunnel or by modifying the gradi- 
ents. Even in cases where such deviations and modifications 
may be impossible, a knowledge of the difficulties lying before 
him would yet greatly aid the engineer in making his 
estimates, 

