308 THS REVOLUTIONS OF THE CRUST OF THE EARTH. 



substances aro rarely found pure, as emploj'etl in the manufacture of 

 glass and jiorcelain ; they are generally mingled together in every i^ro- 

 portion, and form the foundation of fertile soils. According to the ce- 

 ment which gives them their consistency to the period of their forma- 

 tion, and the proportion of their mixture, they form a great variety of 

 rocks which are often difficult to specify, although the petrographic laws 

 controlling the binary mixture of bodies are very simple.* 



The action of the sea depends in a great measure upon the influence 

 of the atmosphere. 'Sow, as the atmosphere of past ages was of neces- 

 sity denser than at the present time, this influence must have been 

 greater. Before concluding from certain general phenomena that the 

 atmosphere diminishes, and with it the action of the sea, we will say a 

 few words upon its composition, which, being that of a mixture, is not 

 invariable. M. A. Smith found that in Northern Scotland the quantity 

 of oxygen contained in the air was the greatest, being 20.999 per 

 cent., and that of carbonic acid the smallest, only 0.033G -per cent. M. 

 Houzeau likewise made, during a long series of years, very exact ob- 

 servations in regard to the variability of the atmospheric elements, and 

 found that the seasons sensibly influenced the proportions of the gases.t 



The observations of M. Menes| had previously made manifest that the 

 quantity of carbonic acid varies with the seasons: In the month of 

 December it undergoes no change ; it increases until the month of May, 

 decreases until the month of August, and reaches its maximum in the 

 month of October. During the night the amount of carbonic acid is 

 greater than during the day; it also increases after an abundant rain. 

 From these facts we conclude that the quantity of carbonic acid in the 

 atmosphere depends directly upon the vital activity of plants and ani- 

 mals ; indeed, they regulate for the most part, if not entirely, its diffu- 

 sion in certain parts of the globe at certain times of the year. 



During the Coal period the atmosphere is supposed to have been 

 especially charged with carbonic acid. This view appears to be corrob- 

 orated by the rapid increase of the coal vegetation and by the remark- 

 able thickness of the beds of carbonate of lime formed by marine ani- 

 mals. If, however, we compare the entire mass of carbon contained in 

 the strata of the Coal period with the carbon at present in the atmos- 

 phere, we find that the latter much surpasses the fossil carbon ; and 

 we must admit with M. Ch. Lyell that the atmosphere of the Coal period 

 differed very little in its composition from that which now surrounds 

 our globe.§ 



In adopting the opinion of the English geologist, we do not wish to 

 infer that the atmosphere is invariable in its elements and in its extent j 

 on the contrary, we are led to believe in a variability both of the mass 



* J. Rotb, Geolog. Zeitschrift, t xvi, p. G75, G02. 



t Comptcs Eendus, t. Iviii, p. 798. 



t Ihid,, t. Ivii, p. 155. 



§ Ch. Lyell, Travels in North America, 1845, vol. i, pp. 150, 152. 



