


ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 341 
into account. A highly permeable rock exposed in section 
may weep so copiously through pores or joints, that it is 
readily broken up when exposed to the atmosphere. 
As it is hardly possible for the engineer to consider the 
lithological character of rocks apart from their geological 
structure, we may briefly indicate how strata may be 
expected to behave in cuttings according as they occupy 
horizontal or inclined positions. 
When homogeneous firm rocks which show very few 
joints are horizontally disposed, they may usually be relied 
upon to be self-sustaining, and to stand with approximately 
vertical faces in any open cutting. Such rocks are permeable 
only to a limited extent, and throw out little or no water. 
Hence, even when they occur as a series of thick beds 
separated by intervening layers of impermeable shales or 
clay, springs are of infrequent occurrence. Where the rocks 
to be cut through consist, on the other hand, of a series of 
highly porous and jointed beds with intervening impermeable 
shales, etc., springs and seepage may be expected. Even 
should water not filter through into the cutting, it is obvious 
that beds of such varying character are sure to weather 
unequally—the softer rocks will crumble away, and constant 
undermining of the overlying beds must take place. Under 
such conditions, it becomes necessary to bench back the 
cutting, or to slope it until the angle of repose is reached. 
But where much water is discharged, the engineer may be 
compelled to mask the cutting with impervious masonry, 
apertures being left in the wall here and there to allow the 
water to escape. 
When strata are excavated in the direction of their dip, 
they can usually be treated as if they were horizontally 
bedded. The point of most importance is the nature of 
the rocks themselves. If the beds are firm and relatively 
impermeable, they may be expected to stand with vertical 
or nearly vertical faces. The chief dangers to be guarded 
against are the escape of water and the action of frost. 
Cuttings made in the direction of the strike usually 
require different treatment. On one side of such an excava- 
tion the beds dip away from the line of cutting, and therefore 
occupy a strong position. Even should they consist of a 
