256 Records ofihe Geological Survey of India. [voL. xin. 
whicli favours chemical action, and because the constituents of the soil are 
further advanced in the path of degradation. 
7. In order to understand the slow chemistry going on in the ground, we 
Origin of cJirbonato of have to conceive the outer shell of the earth generally 
lime alkaline carbonate. covei’ed with more or less vegetable mould, and permeated 
to its greatest known depth by meteoric water. There is no rock, however com¬ 
pact, and there is no depth to which man has penetrated, in which water is not 
found to have permeated by pores, cracks, or fissures. The great agent of change 
is the carbonic acid of the air. This is dissolved in rain water, which also dis¬ 
solves more fi’om the decaying vegetable mould and from the air in the pores of 
the ground, which is rich in this gas. It has two great functions. It attacks 
the silicates of the alkalis and lime, forming carbonates. It further dissolves the 
carbonate of bme, enabling it to be transported by water and to be redeposited 
on evaporation. From the limestone rocks the water takes up carbonate of 
lime and magnesia, which dissolve in its free carbonic acid, and in such forma¬ 
tions it becomes very hard. The amount of carbonate of magnesia dissolved is 
always much less than that of lime. In the silicious rocks the folspathic family 
of minerals decomposes most readily. These consist of silicates of alumina and 
alkali, with generally small quantities of lime and magnesia. The white or soda 
felspar, which contains more soda than potash, is a common ingredient of the 
Himalayan rocks, and the decomposition of this in soils may possibly to some 
extent account for the very great excess of soda over potash salts. The chief 
reason, however, depends on the fact which has been experimentally verified, that 
in a silicate containing both potash and soda the latter is dissolved out with 
greater facility and in much larger quantity than the former. The process of 
decomposition consists in removal of the alkali by the action of carbonic acid, 
while water is taken up, leaving hydrous silicate of alumina or clay. The pre¬ 
sence of alkaline water also assists in promoting the breaking up by dissolving 
some silica. Another group, the lime silicates, is also readily decomposed by the 
action of carbonic acid or alkaline carbonate, and foiuns an additional source of 
carbonate of lime. On the other hand, the talcose rocks, which contain magne¬ 
sian silicate, are hardly attacked at all. This magnesian metamorphosis of 
rock, which is very extensive and very ancient, is also the most permanent, and 
apparently a final one. We have thus accounted for the alkaline carbonate 
and carbonate of lime. The earth water is almost alway.s slightly alkaline, 
and this most important part in the decomposition of the silicious 
rocks and their metamorphosis. The alkaline carbonate rarely, however, appears 
in large amount, because it partly expends itself in decomposing silicate of 
lime, thus forming carbonate of lime, and if free carbonic acid is present, this 
will be dissolved and carried away by the water. If magnesian or lime sulphate 
be present, the carbonate of soda with these will produce lime or magnesian 
carbonate; Avhile sulphate of soda will be found in the solution. It thus happens 
that the waters of the rivers contain apparently no alkaline carbonate, but show 
a permanent neutral re-action. All the river waters, in addition to carbonate of 
lime, which is their chief ingredient, contain also lime and magnesian sulphates 
which there has not been enough alkaline carbonate to decompose. 
