418 



silicates M- This method of estimating the production of carbo- 

 nates from siUcates must however give too small results, 

 because it is highly improbable that all, or even the greater 

 part of the carbonates produced, should be dissolved and carried 

 to the sea. 1 have shown that, as the last trace of free car- 

 bonic acid is utilized in the decomposition of silicates, normal 

 carbonates are formed and nothing is left to dissolve them and 

 carry them away. 



Chamberlin (pp. 563 — 568) gives very good reasons for the 

 opinion that the rate of decomposition of alkaline rocks is 

 mainly dependent on the general elevation of the land. A great 

 elevation must augment the volume and surface of rock exposed 

 to the joint action of water and air and will greatly facilitate 

 the circulation of the underground waters. As periods in the 

 earths history of great general elevation he pronounces among 

 others the Pliocene and Pleistocene. 



The rate of decomposition of silicates must further be 

 governed by the percentage of carbonic acid found in the 

 atmosphere, whereby it acquires a certain regulating influence 

 on the aforenamed percentage. A high percentage of carbonic 

 acid must be accompanied and counteracted by an enhanced 

 rate of combination of the gas with the rocks. 



3. The exhalation of carbonic acid from the interior of 

 the earth is the principal source by which the diminishing 

 factors mentioned are checked. Carbonic acid issues forth 

 from the ground in some places, notably in volcanic environs, 

 in enormous quantities ^), The origin of this carbonic acid is 

 not known. Possibly it is partly derived from the decomposition 



'^] T. Mellard-Reaue (Adresses. Geol. Soc. Live7-j)ool 1876 and 1884, 

 quoted from Chamberlin) estimates this part as corresponding to 

 270 mill, tons of carbonic acid. 



*) Bischof mentions for instance {Lehrbuch der ehem. und phys. Geol. 

 Bd. I p. 691) that a single mineral spring in Nauheim produces yearly 

 about 500 tons of carbonic acid, whereas two others, at Meinberg, yield 

 at least 650 tons pro year. 



