EAKLY ATMOSPHERIC CONDITIONS. 139 







nent state, and thus completed its constitution merely as a 

 planetary body. 



4. The FOURTH stage was characterized mainly by aerial de- 

 velopments and changes. It embraces that vast period during 

 which the rocks of the Cambrian, Silurian, Old Red Sandstone, 

 and Carboniferous systems were formed. At the commence- 

 ment of this period, the atmosphere must of necessity have 

 been in an exceedingly crude and impure state. Besides other 

 gross and noxious elements, it must have borne in its bosom 

 all, or nearly all, of the carbonic acid gas which subsequently 

 became condensed in the mountain limestone and various 

 other limestone deposits, and the carbon of which, parting with 

 its oxygen, became embodied in the immense beds of mineral 

 coal, found, more or less, in almost every quarter of the earth. 

 An atmosphere thus surcharged with this noxious vapor, must 

 have been incompatible with the existence of any forms of 

 organic life, except those of a low order; and accordingly we 

 find that the plants and animals of this vast period were, as 

 shown by their fossil remains, exclusively such as inhabited 

 the ocean and the marshy and frequently submerged places in 

 its vicinity situations intermediate between the properly 

 marine and the properly terrestrial. 



It was, doubtless, owing mainly, if not wholly, to atmos- 

 pheric causes that the solar rays during this period had but 

 little influence upon the surface of the earth, and that a nearly 

 uniform temperature prevailed at all latitudes and at all sea- 

 sons. Geologists have usually attempted to account for the 

 high degree and general uniformity of this temperature, as 

 indicated by the universally tropical nature of the plants and 

 animals of this period, by referring it to a radiation of the 

 internal heat of the earth, which it is supposed must, at that 

 early period, have been much more intense than in subsequent 



