HVPOTHESTS OF “CAUSE OF GLACIAL PERIODS 557 
characterized as carbonation, and consists essentially of the sub- 
stitution of carbonic acid for the silicic acids. The carbonic 
acid is derived, in the main, from the atmosphere. In their 
soluble condition, the carbonates so formed are largely bicar- 
bonates. This is the only form in which calcium carbonate is 
appreciably soluble, and the same is true in a less degree of mag- 
nesium carbonate. The monocarbonates of potassium and sodium 
are highly soluble, but the bicarbonates also appear in solution. 
Practically the carbonates of these alkalis usually become 
changed into other salts (sulphates, chlorides, etc.) and the car- 
bon dioxide that may have been temporarily locked up with 
them is set free by the change, or enters some other combina- 
tion. The magnesium and calcium carbonates are also in part 
changed to other salts. But for the purposes of this discussion, 
which is concerned chiefly with the final issue and not with the 
transient stages of these compounds, it is sufficient to note that 
the chief result of the decomposition of the original silicates is 
the formation of calcium and magnesium carbonates, which are 
deposited as limestones and dolomites and thus lock up carbon 
dioxide at the expense of the atmosphere. The amount so taken 
from the air in the known geological periods has been variously 
estimated at from 20,000 to 200,000 times the present content ; 
indeed estimates have gone beyond the last figure. When it is 
considered that 44 per cent. of all pure limestone and a higher 
per cent. of all pure dolomite is carbon dioxide, it 1s obvious that 
the total quantity is very large, and its computation is dependent 
upon the estimate of the total amount of limestone and dolomite 
IMthe crust Of, the: earth, 
A second source of permanent loss consists of the consump- 
tion of carbonic acid by plants and the fixation of the carbon in 
carbo-hydrates, hydro-carbons and other carbonaceous compounds 
which ultimately take the form of coals, bitumens, oil, gas, and 
perhaps most important of all, disseminated organic matter in 
the sedimentary series. 
Exceptions—Some hydro-carbons have probably been pro- 
duced by inorganic action, notably those derived from carbides 
