
a a ae 

370 STRUCTURAL AND FIELD GEOLOGY 
gradually lowered, the loose materials travelling outwards 
from the scene of their origin to the borders of the desert. 
Action of Rain.—Even in countries like our own, insola- 
tion and deflation doubtless share in the disintegration of 
rocks and the transport of the loosened materials. Un- 
doubtedly, however, in these latitudes, the most conspicuous 
agents employed in the work of reducing rocks to the 
condition of grit, sand, and clay, are rain and frost. Rain 
always contains some oxygen and carbonic acid absorbed 
from the atmosphere, and after it reaches the ground, still 
larger stores are derived by it from the decaying vegetable 
and animal matter with which soils are more or less impreg- 
nated. It is thus enabled to attack the minerals of which 
rocks are composed. In every region, therefore, where rain 
falls, soluble rocks, such as limestone, are gradually dissolved, 
while other kinds of rock are decomposed and disintegrated. 
In limestone areas it can be shown that sometimes hundreds 
of feet of rock have thus been gradually removed from the 
surface of the land. And the great depth now and again 
attained by rotted rock testifies likewise to the chemical 
activity of rain-water. This is particularly noticeable in 
warm-temperate, sub-tropical, and tropical latitudes, where 
felspathic rocks are not infrequently decomposed to depths of 
a hundred feet or more. In temperate and northern regions, 
the amount of rotted rock is rarely so great. The thicker 
rock-crusts of southern latitudes are supposed to be due to 
the larger supplies of acid derived from the more abundant 
vegetation. To some extent this is probably true, and there 
can be little doubt, also, that the chemical action of rain is 
facilitated by the higher temperature of those regions. There 
is another reason, however, for the relatively meagre develop- 
ment of rotted rock in northern countries generally. Those 
regions have, at a geologically recent date, been subjected to 
glacial conditions. Broad areas of temperate Europe and 
North America, for example, have been scraped bare by 
extensive ice-sheets, resembling those of Greenland and the 
Antarctic Circle. In more southern latitudes the rotted rocks 
have escaped such abrasion and denudation, and hence it is 
not strange that they should attain so great a thickness. 
The decomposed rock-material encountered in the northern 

