, but the walls of the cavity
consist partly of a prominent rim of sandy mud, formed of the matter
which has been forcibly expelled from the pit. All the cavities having
an oval form are deeper at one end, where they have also a higher rim,
and all the deep ends have the same direction, showing towards which
quarter the wind was blowing. Two or more drops are sometimes seen to
have interfered with each other; in which case it is usually possible to
determine which drop fell last, its rim being unbroken.

[Illustration: Fig. 13.

Recent rain-prints, formed July 21, 1849, at Kentville, Bay of Fundy,
Nova Scotia. The arrow represents the direction of the shower.]

On some of the specimens the winding tubular tracks of worms are seen,
which have been bored just beneath the surface (see fig. 13, _left
side_). They occasionally pass under the middle of a rain-mark, having
been formed subsequently. Sometimes the worms have dived beneath the
surface, and then reappeared. All these appearances, both of rain-prints
and worm-tracks, are of great geological interest, as their exact
counterparts are seen in rocks of various ages, even in formations of
very high antiquity.[266] Small cavities, often corresponding in size to
those produced by rain, are also caused by air-bubbles rising up through
sand or mud; but these differ in character from rain-prints, being
usually deeper than they are wide, and having their sides steeper.
These, indeed, are occasionally vertical, or overarching, the opening at
the top being narrower than the pit below. In their mode, also, of
mutual interference they are unlike rain-prints.[267]

In consequence of the effects of mountains in cooling currents of moist
air, and causing the condensation of aqueous vapor in the manner above
described, it follows that in every country, as a general rule, the more
elevated regions become perpetual reservoirs of water, which descends
and irrigates the lower valleys and plains. The largest quantity of
water is first carried to the highest region, and then made to descend
by steep declivities towards the sea; so that it acquires superior
velocity, and removes more soil, than it would do if the rain had been
distributed over the plains and mountains equally in proportion to their
relative areas. The water is also made by these means to pass over the
greatest distances before it can regain the sea.

It has already been observed that in higher latitudes, where the
atmo