16 GEOLOGICAL FORMATIONS. 



mation. We would after this descend, in succession, through 

 strata of limestone, called the mountain or carboniferous 

 limestone ; through what is called the Old Red Sandstone, and 

 thence through what is known as the Silurian and Cambrian 

 systems of deposits. These stratifications, taken together, 

 have been estimated by Dr. John Pye Smith, as measuring a 

 thickness of not less than one hundred and thirty thousand 

 feet. They abound with fossils which, with perhaps slight 

 exceptions, and these confined to their higher portions, are 

 exclusively marine ; and the character and magnitude of some 

 of these, and their invariableness of size and constitution as 

 they occur in all latitudes, show that a high and unvarying 

 temperature prevailed on all parts of our globe during the 

 period when they flourished, which could not have depended, 

 in any great degree, upon the solar rays, but is generally sup- 

 posed to have been caused by radiations from subterranean 

 fire, then more intense than at subsequent periods. This 

 whole series of stratifications has been called the Transition 

 Formation, comprising, in the period of its production, those 

 changes in the physical conditions of the earth's surface, which 

 were necessary to qualify it for the production of terrestrial 

 vegetation and the healthy sustenance of air-breathing animals. 

 This completes the enumeration of the fossiliferous stratifi- 

 cations, which, according to some estimates, are of an aggre- 

 gate thickness exceeding twenty miles ! These all, including 

 the remains of the plants and animals which subsisted during 

 their respective epochs, were quietly deposited at the bottoms 

 of oceans, estuaries, and lakes, and subsequently consolidated 

 and petrified, and thus, as faithful records of the natural his- 

 tory of our planet, they have been preserved through the 

 untold ages which have elapsed from the period of their living 

 existence until present time ! 



