CONDITION OF THE FLOOR OF THE OCEAN. 323 



and it is well known that the acid silicates have a lower specific gravity 

 than the intermediate or basic ones. B3' a continual repetition of this 

 process the continental protuberances have been gradually built up of 

 lighter materials than the other parts of the lithosphere. The rela- 

 tively light quartz, which is also the most refractory, the most stable, 

 and the least fusible among rock-forming minerals, plays in all this 

 the principal role. The average height of the surface of the conti- 

 nents is about 3 miles above the average level of the abysmal regions. 

 If now we assume the average density of the crust beneath the conti- 

 nents to be 2.5, and of the part beneath the abysmal regions to be 3, 

 then the spheroidal surface of equal pressure — the tektosphere — 

 would have a minimum depth of 18 miles beneath the continents and 

 15 miles beneath the oceans, or, if we assume the density of the crust 

 beneath the continents to be 2.5, and beneath the abysmal regions to 

 be 2.8, then the tektosphere would be 28 miles beneath the continents 

 and 25 miles beneath the oceans. The present condition of the 

 earth's crust might be brought about by the disintegration of a 

 quantit}" of quartz-free volcanic rock, covering the continental areas 

 to a depth of 18 miles, and the reformation of rocks out of the dis- 

 integrated materials. 



When the lighter and more bulky substances have accumulated there 

 has been a relative increase of volume, and in consequence bulging 

 has taken place at the surface over the continental areas. Where the 

 denser materials have been laid down there has been flattening, and in 

 consequence a depression of the abysmal regions of the ocean basins. 

 It is known that, as a general rule, where large masses of sediment 

 have been deposited, their deposition has been accompanied by a 

 depression of the area. On the other hand, where broad mountain 

 platforms have been subjected to extensive erosion the loss of altitude 

 by denudation has been made good by a rise of the platform. This 

 points to a movement of matter on to the continental areas. 



If this be anything like a true conception of the interactions that 

 are taking place between the various geospheres of which our globe is 

 made up then we can understand why, in the gradual evolution of 

 the surface features, the average level of the continental plains now 

 stands permanently about 3 miles a^30ve the average level of those 

 plains which form the floor of the deep ocean basins. We may also 

 understand how the defect of mass under the continents and an 

 excess of mass under the oceans have been brought about, as well as 

 deficiency of mass under mountains and excess of mass under plains. 

 Even the local anomalies indicated by the plumb line, gravity, and 

 magnetic observations may in this way receive a rational explanation. 

 It has been urged that an enormous time — greater even than what 

 is demanded by Darwin— would be necessary for an evolution of the 

 existinsr surface features on these lines. I do not think so. Indeed, 



