THE PERMIAN PERIOD. 663 



much elucidated by direct experimentation as yet, and while the reactions are 

 very difficult to anticipate theoretically, it seems permissible, in the present 

 state of knowledge, to suppose that this process might go forward until the car- 

 bonates of the ocean were well nigh or quite exhausted. Even if completely 

 exhausted, the calcium necessary for organic life could probably be supplied 

 by the calcium sulphate, which is roundly ten times as abundant as the carbonate 

 in the present ocean. Indeed, it has sometimes been urged that it is the cal- 

 cium sulphate that is actually used by life, instead of the carbonate, but this 

 does not appear to be sufficiently supported by experimental determinations. 

 If calcium sulphate is converted by organisms into the calcium carbonate of 

 their shells, skeletons, etc., the sulphuric acid set free probably unites with the 

 bases of the carbonates of the water — except so far as converted into sulphides 

 and deposited as such — and sets their carbonic acid free, so that the ultimate 

 result of the reaction is much the same whether the organisms use calcium car- 

 bonate directly or calcium sulphate. This supposed depletion of the oceanic 

 carbonates, even if it began Avith an ocean fully saturated by them and ended 

 with complete exhaustion, would only help to solve the problem of the supply 

 of carbon dioxide for the great limestone formations of the period, though the 

 help would seem to be very important. But the depletion of the ocean con- 

 tent was probably a very radical feature in the atmospheric problem, to which 

 this discussion is preliminary. 



The rest of the solution of the problem of an adequate source for the carbon 

 dioxide of the Subcarboniferous limestones is probably found in two other quarters. 

 The limestone of the period was certainly not all original, in the sense of having 

 been formed from the silicates of the crystalline rocks. A notable part of it was 

 derived from the exposed portions of limestones previously formed. A comparison 

 of the map of the period (Fig. 228) with the maps of previous periods (Figs. 95, 

 129, 174, 195, etc.), shows that not a little of the exposed land was formed cf 

 the older Paleozoic sediments, among which limestones were a notable element. 

 Probably the lime in the previous sediments was not greater per unit area, on 

 the average, than in the crystalline rocks, since the sediments were derived from 

 the crystalline rocks. It was perhaps somewhat less, since there was some loss 

 of lime to the ocean; but it was probably more readily removed. This remova) 

 involved the combination of a portion of the free carbon dioxide of the arr with 

 the limestone to form bicarbonate, which was thus dissolved and carried down to 

 the sea. Thus a portion of the carbon dioxide was temporarily locked up, but 

 when the lime was secreted by the sea organisms, this second equivalent of the 

 carbon dioxide was set free, and was subject to diffusion into the atmosphere, 

 where it was available for a repetition of the process. A comparatively limited 

 amount of free carbon dioxide might thus serve, in time, as the carrier of a 

 large amount of carbonate from the land to the sea. It is difficult to esti- 

 mate the portion of the Subcarboniferous limestone which was thus derived 

 from previous limestones, and the portion which was added by the original 

 carbonation of the silicates of crystalline rocks, for the data are very inade- 

 quate. 



