The upper part of the
mountain is supposed to be much more porous than its base, and the plane
which divides the porous from the non-porous rock, to cut the surface of
the mountain along the line, A, N, M, B, C, D, E, F. The highest point of
the plane is F, and the lowest point A. The effect of rain upon the model
fig. 2 would be, to wet its upper half: water would ooze out along the
whole of the lines A, N, and M, B, C, D, E, F; and there would be a small
fountain at A, and a large one at M. But in the actual mountain, fig. 1,
we should not expect to find the same regularity as in the model. The
rind of the earth, with its vegetation and weather-impacted surface,
forms a comparatively impermeable envelope to the mountain, not likely to
be broken through, except at a few places. But ravines, such as r, would
be probably denuded of their rind, and there we should find a line of
minute fountains at the base of the porous rock. If there be no actual
fountains, there would at least be some vegetation that indicated
dripping water: thus the appearance is well known and often described, of
a ravine utterly bare of verdure above, but clothed with vegetation below
a sharply defined line, whence the moisture proceeds that irrigates all
beneath. We should also be almost certain of finding a spring breaking
forth near m or even near a. But in the valley V we should only see a few
signs of former moisture, along e, f; such as bunches of vegetation upon
the arid cliff, or an efflorescence of salts.
Pages:
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307