30 PRINCIPLES OF STRATIGRAPHY 



to refrigerate the climate. Tlie amount of COo taken from the air 

 in the known geologic periods and locked up in the coals and lime- 

 stones of the earth has been variously estimated at from 20,000 to 

 200,000 times the present atmospheric content, or even more. An- 

 other factor favoring the consumption of COo is the extensive sub- 

 'terranean decomposition of the rocks following a period of elevation 

 with its attendant fissuring of the rock and the deepening of the 

 zone of circulation of surface waters. These, supplied with COo at 

 the surface, are active in the carbonation of the decomposing rocks. 

 The solution of limestones and other carbonates is a further source 

 of abstraction of COo from the atmosphere, since the original mono- 

 carbonates are changed to bicarbonates in the process of solution, the 

 second molecule of COo being derived from the atmosphere. This 

 is, however, a temporary abstraction, for on the redeposition of the 

 limestone, either by chemical or organic agencies, the second mole- 

 cule of COo is set free again. Hence periods of extensive limestone 

 deposition must be periods of extensive supply of COo to the atmos- 

 phere, and this would have the effect of ameliorating the climate. 

 It is important to note that extensive coal formation is dependent on 

 the great expanse of land areas, while extensive limestone formation 

 depends on the expanse of the sea. Extension of the land is accom- 

 panied further by an extension of the deep-seated circulation, and by 

 decomposition and carbonation of the crystalline rocks, as well as by 

 periods of solutiqn of the carbonates. The two periods of great 

 land expansion, followed by well-authenticated glacial periods, are 

 the close of the Palaeozoic and that of the Cenozoic. Other periods 

 are the pre-Cambric, the close of the Siluric and the opening of the 

 Devonic, the close of the Jurassic and opening of the Comanchic, 

 and the end of the Cretacic. Periods of marine extension, on the 

 other hand, with the formation of much limestone and the accom- 

 panying setting free of CO,, were the middle and early upper 

 Ordovicic, the early Siluric (Niagaran), the Mississippic, the upper 

 Jurassic, the mid-Cretacic, and in some regions the Eocenic and 

 Oligocenic. ]\Iuch evidence exists that these periods of extensive 

 limestone formation were periods of mild and equable climates, very 

 nearly uniform for all latitudes. 



Distributio}i of Heat JVithin the Earth's Atmosphere. The sun's 

 heat is distributed through the atmosphere by radiation, conduction, 

 and convection. The heat which the earth receives from the sun is 

 returned to the atmosphere by radiation. These non-luminous heat 

 waves are much more readily absorbed by the air than the luminous 

 ones which reach it from the sun. The atmosphere in contact with 

 the land is further heated by conduction, provided the surface of the 



