
Parr I. Sect. ii. § 1.] RAIN-ACTION. 341 
- comparatively soon be exhausted and would cease to support the 
same vegetation. This result indeed occurs partially, especially on flat 
lands, but would be far more widespread were it not that rain, 
gradually washing off the upper part of the soil, exposes what lies 
_ beneath to further disintegration. ‘This removal takes place even on 
grass-covered surfaces through the agency of earth-worms, by which 
fine particles of loam are brought up and exposed to the air to be 
dried and blown away by wind or washed down by rain. The lower 
limit of the layer of soil is thus made to travel downward into 
_ the subsoil, which in turn advances into the underlying rock. As 
- Hutton long ago insisted, the superficial covering of soil is constantly, 
though slowly, travelling to the sea. In this ceaseless transport 
rain acts as the great carrying agent. The particles of rock and of 
soil are step by step moved downward over the face of the land till 
they reach the nearest brook or river, whence their seaward progress 
may be rapid. A heavy rain discolours the water-courses of a 
country, because it loads them with the fine débris which it removes 
from the general surface of the land. In this way rain serves as the 
means whereby the work of the other disintegrating forces is made 
conducive to the general degradation of the land. The decomposed 
crust produced by weathering, which would otherwise accumulate — 
over the solid rock and in some measure protect it from decay, is 
removed by rain, and a fresh surface is thereby laid bare to further 
decomposition. | 
Unequal Erosive Action of Rain.—While the result of 
rain action is the general lowering of the level of the land, this 
process necessarily advances very unequally in different places. On 
flat ground the waste may be quite inappreciable except after long 
intervals and by the most accurate measurements, or it may even give 
place to deposition, the fine detritus washed off the slopes being 
spread out so as actually to heighten the alluvial surface. In 
numerous localities great variations in the rate of erosion by rain 
may be observed. ‘Thus, from the pitted, channelled ground Jying 
immediately under the drip of the eaves of a house, fragments of 
stone and gravel stand up prominently, because the earth around and 
above them has been washed away by the falling drops, and because, 
being hard, they resist the erosive action and screen the earth below 
them. Ona larger scale the same kind of operation may be noticed 
in districts of conglomerate, where the larger blocks, serving as a 
protection to the rock underneath, come to form as it were the 
capitals of slowly-deepening columns of rock (Fig. 93). In certain 
valleys of the Alps a stony clay is cut by the rain into pillars, each 
of which is protected by, and indeed owes its existence to, a large 
block of stone which lay originally in the heart of the mass (Fig. 94). 
These columns are of all heights, according to the positions in which 
the stones may have originally lain. 
There are instances, however, where the disintegration has been 
1 Theory of the Earth, Part II. Chaps. V., VI. 
