

342 STRUCTURAL AND FIELD GEOLOGY 
series of alternating porous and impermeable rocks, they will 
often stand with a vertical face. The reason is obvious, for 
any water the porous beds may contain will tend to escape 
downwards along the planes of bedding—it is drained away 
from the cutting. On the opposite side of the excavation, 
the strata dip into the cutting and therefore occupy an 
unstable position. The tendency is for the truncated beds 
to slide downward; and should they consist of alternating 
pervious and impervious beds, springs will come out, under- 
mining will take place, and piecemeal or wholesale collapse 
must result. Rocks occupying such a position must be built 
up, care being taken to allow passage for the percolating 
water. 
Excavations in massive igneous rocks will often stand 
with vertical or approximately vertical faces. The character 
of the jointing, however, has to be carefully considered, and 
the possible action of springs, in undermining and dislodging 
masses, and the general effect of frost, must not be over- 
looked. Schistose rocks, in like manner, are often firm and 
stable when opened up, more especially if the excavation runs 
in the same direction as the dip of the foliation. But when 
the cutting traverses such rocks along the strike, they are apt 
to behave much in the same way as sedimentary strata—on 
one side of the excavation they may be expected to stand 
well; on the opposite side slips and falls are likely to take 
place. Their stability, moreover, is often affected by the 
very irregular jointing, and by the variable character of the 
rocks themselves; so that while some schists readily allow 
the passage of water, others are more or less impermeable. 
The stability of this class of rocks further depends, to some 
extent, upon the nature of the foliation. Evenly foliated 
rocks, which simulate ordinary sedimentary strata, may be 
expected to behave much in the same way as the latter. 
When schists are much crumpled and contorted, however, 
the individual folia are more securely locked together, and 
slipping is much less likely to take place, so that such rocks 
may often be treated as if they were highly jointed igneous 
masses. Slates, it need hardly be said, more closely resemble 
steeply bedded sedimentary rocks, such as greywackés and 
shales, the stability of the faces of a cutting depending upon 

