ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 459 



periods, is in itself more than sufficient to take up all the oxygen 

 which the atmosphere of the present day contains.* He calcu- 

 lates also that a stratum of carbon, spread over the whole surface 

 of the globe, 2.6 feet thick, would be sufficient to convert all the 

 oxygen of the atmosphere into carbonic acid; and after consider- 

 ing how richly furnished the sea is with animal and vegetable life, 

 how rich its sedimentary deposits must be in organic substances ; 

 that the earlier sedimentary rocks are highly changed with car- 

 bonaceous and bituminous substances, that beds of coal and 

 lignite are spread over an area of many hundreds of square 

 miles with a very considerable average thickness, he comes to 

 the conclusion, that a layer 2.6 feet thick is far from being an 

 equivaleut to all the carbon existing in the earth, leaving 

 altogether out of the question the carbon of the organic world 

 on its surface. This opinion certainly seems to be well grounded, 

 and there would appear to be just reason for supposing 

 that the oxygen of the atmosphere existed originally in the- 

 state of carbonic acid, and that a considerable quantity of 

 carbon, besides that which was in combination with the oxygen^ 

 must have existed in the original atmosphere, either free, or in 

 combination with other elements, and probably especially with 

 hydrogen. Thus the gaseous envelope of the original globe 

 must have been an enormous atmosphere of water, carbonic acid, 

 carburetted hydrogen, and nitrogen, together with comparatively 

 small quantities of sulphurous acid, and sulphuretted hydrogen, 

 hydrochloric acid and metallic chlorides. The pressure of such an 

 atmosphere must have been prodigious, at least 100 times greater 

 than that of the preseut time ; and in conjunction with its compo- 

 sition sufficient to produce effects totally different from those 

 caused by atmospheric influences at the present day. Among its 

 most remarkable properties must have been its power of absorb- 

 ing heat. Dr. Hunt has shown that the atmosphere of paleozoic 

 times must, from the amount of carbonic acid in it, have greatly 

 aided to produce the elevated temperature then existing.^ 

 How much more must this have been the case when the atmos- 

 phere contained such hydro carbons as marsh and olefiant gases, 

 whose power of absorbing radiant heat greatly exceeds that of 

 carbonic acid .J Dr. Hunt has indeed indicated the part which such 

 hydrocarbons may thus have played. After the fluid globe had suffi- 



* Chem. and Phys. Geologie, ii. p. 35. 

 f Canadian Naturalist, vol. viii. p. 324. 

 t Tyndal : Heat considered as a mode of motion, p. 362. 



