
356 STRUCTURAL AND FIELD GEOLOGY .- 
with those vertical wall-like intrusions, known as dykes (see 
p. 202). When dykes cut across inclined rocks in the general 
direction of the strike, they naturally act as subterranean 
dams, interrupting the underground water flowing towards 
them, and forcing it to risé to the’ surface (Figs agg 
Sta : 

FIG. 133.—INTERCEPTION OF UNDERGROUND DRAINAGE BY DYKE. 
Junction-springs of this kind are very common in Scotland. 
Probably the most important springs of all, however, are those 
that appear on lines of faulting or dislocation. These, as we 
have learned, frequently bring permeable against impermeable 
rocks, and many of the largest dislocations run in the direction 
of the strike of inclined strata. Hence, when a series of 
porous strata dip at a low angle for a long distance, towards 
one of these faults, on the other side of which the rocks are 
more or less impermeable, all the conditions favour the 
formation of strong springs (Fig. 134). Even when the strata 

FIG. 134.—INTERCEPTION OF UNDERGROUND DRAINAGE BY FAULT, 
on both sides of a fault are porous, springs will usuaily 
indicate its presence, for faults are often filled or lined with 
clay, etc., and thus form more or less impervious barriers, 
while the adjacent rocks, commonly much fissured and frac- 
tured, afford the underground water a ready passage to the 
surface. A fault traversing horizontal strata in such a way 
as to bring permeable and impermeable rocks into juxta- 
position likewise causes springs to appear, whenever the 

