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‘344 | DYNAMICAL GEOLOGY. _—— [Boox Ith = 
§ 2, Underground Water. 
A great part of the rain that falls on land sinks into the ground ~ 
and apparently disappears; the rest flowing off into runnels, brooks, 
and rivers, moves downward to the sea. It is most convenient to 
follow first the course of the subterranean water. 
All rocks being more or less porous, and traversed by abundant 
joints and cracks, it results that from the bed of the ocean, from the 
bottoms of lakes and rivers, as well as from the general surface of 
the land, water is continually filtering downward into the rocks 
beneath. To what depth this descent of surface water may go is 
not known. As stated in a former section, it may reach as far as 
the intensely heated interior of the planet, for, as the already 
quoted researches of Daubrée have shown, capillary water can — 
penetrate rocks even against a high counter-pressure of vapour 
(ante, p. 299). Probably the depth to which the water descends — 
varies indefinitely according to the varying nature of the rocky 
crust. Some shallow mines are practically quite dry, others of 
great depth require large pumping engines to keep them from being 
flooded by the water that pours into them from the surrounding 
rocks. Yetas a rule, the upper layers of rock in the earth’s crust are 
fuller of moisture than those deeper down. 
Underground Circulation and Ascent of Springs.— 
The water which sinks below ground is not permanently removed 
from the surface, though there must be a slight loss due to ab- 
sorption and chemical alteration of rocks. Finding its way through 
joints, fissures, or other divisional planes of rocks, it issues once more 
at the surface in springs. This may happen either by continuous 
descent to the point of outflow or by hydrostatic pressure. In the 
former case, rain-water sinking underneath, flows along a subterranean 
channel until, when that channel is cut by a valley or other de- 
pression of the ground, the water emerges again to daylight. Thus 
a 

Fig. 95.—SIMPLE oR SURFACE SPRINGS. 
in a district having a simple geological structure (as in Fig. 95), a 
sandy porous stratum (e), through which water readily finds its 
way, may rest on a less easily permeable clay (d), followed under- 
neath by a second sandy pervious bed (c), resting as before upon 
comparatively impervious’ strata (a). Rain falling upon the upper 
sandy stratum (¢), will sink through it to the surface of the clay (d), 
' This term impervious must evidently be used in a relative and not in an absolute 
sense. A stiff clay is practically impervious to the trickle of underground water; hence 
its employment as a material for puddling (that is, making water-tight) canals and 
reservoirs. But it contains abundant interstitial watcr, on which indeed its characteristic 
plasticity depends. 
