XI THE GEOLOGICAL RECORD 217 



so, I think, the next great advance was due to a continua- 

 tion of the same process by a different agency. Geologists 

 have often remarked on the progressive increase in the 

 proportion of limestone in the later than in the earlier 

 formations. In our own country we see a remarkable 

 abundance of limestone during the Secondary era, as shown 

 in our Lias, Oolites, Portland stone, and Chalk rocks ; and 

 somewhat similar conditions seem to have prevailed in 

 Europe, and to a less extent in North America. As lime- 

 stone is generally a carbonate of lime, it locks up a consider- 

 able amount of carbon which might otherwise increase the 

 quantity of carbonic acid in the atmosphere ; and as lime, 

 or its metallic base, calcium, must have formed a considerable 

 portion of the original matter of the earth, solid or gaseous, 

 the continued formation of limestone through combination 

 with the carbonic acid of the atmosphere must have led to 

 the constant diminution of that gas in the same way that 

 the formation of coal reduced it. 



It seems probable that when the earth's surface was in a 

 greatly heated condition, and no land vegetation existed, the 

 atmosphere contained a much larger proportion of carbon- 

 dioxide than at present, and that a continuous reduction of 

 the amount has been going on, mainly through the extrac- 

 tion of carbon from the air by plants and from the water by 

 marine animals and by chemical action. The superabundance 

 of this gas during the early stages of the life-world facilitated 

 the process of clothing the land with vegetation soon after it 

 appeared above the waters ; while its absorption by water 

 was equally useful in rendering possible the growth of the 

 calcareous framework or solid covering of so many marine 

 animals. 



With the progressive cooling of the earth and the 

 increased area of land-surface, more and more of the atmo- 

 spheric carbon became solidified and inactive, thus rendering 

 both the air and the water better fitted for the purposes of 

 the higher, warm-blooded, and more active forms of life. 

 This process will, I think, enable us partially to understand 

 the fundamental changes in life-development which character- 

 ised the three great geological areas ; but it does not seem 

 sufficient to explain the very sudden and complete changes 



