THE FUTURE OF THE DRY LAND. 249 



a reassuring figure to us. But the geologist, who looks at the past 

 as well as at the future, far beyond existing generations, can 

 draw more than one lesson from it. First, the whole history of 

 the globe not being included in a space of time relatively so short, 

 the result teaches us that its equilibrium has more than once been 

 troubled by great phenomena of dislocation, too rare, however, for 

 it to be possible for man to have been a witness of them, which, 

 building up new reliefs as barriers to destruction, have given 

 new impulses to the action of natural forces. 



On the other hand, geological observations furnish a tolerably 

 approximate measure of the maximum thickness of the deposits 

 that are made in the bottom of the sea. The total thickness 

 amounts, according to Dana, to 45,000 metres. To learn how long 

 a time may have been occupied with the formation of such de- 

 posits, let us seek to represent to ourselves what now becomes of 

 the products of the destruction of the continents. 



These deposits, it is now known, do not extend, by a great deal, 

 over the whole surface of the sea bottom ; but they form a zone of 

 strata which the deep-sea sounding expeditions have enabled us 

 to define fairly well. According to Mr. John Murray's estimate, 

 the sediments formed by the destruction of the continents spread 

 themselves over about a fifth of the oceanic surface. Thus, al- 

 though the oceanic area is superior to the surface of the land, the 

 mass of the deposits distributing themselves over only a fraction 

 of the extent, there may result, at the end of 5,000,000 years, an ac- 

 cumulation of sediments capable of forming a body 750 metres 

 thick. But this thickness would certainly be very unevenly dis- 

 tributed; almost null at the finishing point of the deposits in 

 breadth, its thickness would be much greater near the coasts, and 

 it would not be hazardous to suppose that it might rise there to 

 2,000 or 3,000 metres. To realize the total thickness of 45,000 

 metres that is, to explain geological history it would be sufficient 

 to suppose that the life of the globe has included some fifteen or 

 twenty periods of 4,500,000 years, or from 67,000,000 to 90,000,000 

 years, a number a little less than the 100,000,000 years which Sir 

 William Thomson has calculated upon estimates of the loss of 

 internal heat. 



The objection may be brought up that I have neglected in this 

 calculation the contributions of volcanic action to the land relief, 

 which it is thought should be counted in attenuation of the de- 

 structive effect of running waters. We owe to Cordier a calcula- 

 tion that the lavas which have been thrown up during the his- 

 torical period represent at most 500 cubic kilometres, or, counting 

 that period at 3,000 years, a sixth of a cubic kilometre per year. 

 This is a very little affair compared with the amount of the waste 

 which I have pointed out. We should likewise recollect that 



