
ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 355 
mass of granitoid rock is represented cutting across a series 
of relatively impervious strata. The rain passes downwards 
through the much-jointed eruptive mass, where it accumulates. 
The water cannot escape laterally, because it is dammed back 
by the impervious beds (0); it therefore continues to accumu- 
—— =. 
g ae ‘ Pye 2s ~ Ss : 
> = a Sr Glee ee 
ZEEE a peat ma A 
ae LEZ is aa —_ —— ~ oes ae ~ 
EZ Z222 - We y 2 ~ 
EEZEALZZA Ze Se a Ss 
LZzZLZ hs 3 Le a 
LAZZEAD EB Ss ™“. aw 
Zz Zz BEES cord —— 
FIG. 131.—HEAPING-UP OF WATER IN IGNEOUS ROCK. 
late until it reaches the point where the line of junction 
comes to the surface. Here, under hydrostatic pressure, it 
flows out as a more or less copious spring (s) or line of 
springs. In many cases, however, the water is derived from 
the stratified rocks rather than the intrusive mass by which 
they are intersected. In Fig. 132, the strata, consisting of 

FIG. 132.—INTERCEPTION OF UNDERGROUND DRAINAGE BY 
INTRUSIVE ROCK. 
pervious and impervious rocks, dip towards the igneous mass. 
Water soaking through the porous beds (f) is dammed back 
by the basalt (4) and forced to the surface (s) along the 
junction-line. Now and again, the water discharged under 
such conditions would seem to come from the rocks on both 
sides of the junction, especially when the igneous mass is 
of the nature of a batholith, and exposed at the surface over 
a considerable area. 
Springs, as we have already seen, are often associated 
