Geology and Natural History. 339 



about five miles. (3) Folding by lateral pressure commences at a 

 depth of about five miles, and gradually increases, being greatest 

 near the surface of the earth." Furthermore, 



4. " Within certain limits, it is true that the depth of the 

 unstrained surface increases as the square root of the time that 

 has elapsed since the consolidation of the globe." Also, 



5. " Folding by lateral pressure was effected most rapidly in 

 the early epochs of the earth's history, and, since then, the total 

 amount of rock folded in any given time decreases nearly in pro- 

 portion as the square root of the time increases. The same law 

 being approximately true of the total amount of rock stretched 

 by lateral tension, it follows that the ratio of the amount of rock 

 stretched to the amount folded in a given time is very nearly 

 constant, but in reality slightly diminishing as the time increases." 



While not claiming that great weight should be attached to the 

 numerical results obtained, the author goes on to consider the 

 effects of crust-stretching and folding on the evolution of the 

 earth's surface features. Assuming that the formation of the great 

 continental masses took place in the initial period of the earth's 

 history, it follows that : 



6. " Owing to the pressure of the continental wrinkles, the 

 amount of stretching under them must have been very much less 

 thau under the great oceanic areas. Thenceforward, therefore, 

 crust-stretching by lateral tension must have taken place chiefly 

 beneath the ocean-basins, deepening them and intensifying their 

 character. And, in leading to the continual subsidence of the 

 ocean-bed, it is evidently a physical cause of the general perma- 

 nence of oceanic areas; a cause, it is true, continually receding 

 from the surface, and diminishing in intensity with the increase 

 of time, but probably even now not quite ineffective. 



" Again, the amount of crust-stretching by lateral tension being 

 greatly in excess of the amount of crust-folding by lateral pres- 

 sure due to secular cooling, it follows that folding beneath the 

 ocean-bed will do little but diminish its rate of subsidence. The 

 effects of folding in changing the form of the earth's surface 

 features will therefore be most apparent in the continental areas, 

 especially in those regions where the change of vertical pressure 

 above the folded layers diminishes most rapidly, i.e., near the 

 coast-lines where the slope toward the ocean depths is greatest. 

 It is perhaps worthy of remark that these are the districts where 

 earthquake and voleanic action are now most prevalent. In the 

 coast regions, moreover, the products of continental denudation 

 are chiefly deposited, and the rock-folding due simply to secular 

 cooling produces in vast masses of sediment still more crushing 

 and folding. The direction of the folds will be perpendicular 

 to the average slope of the surface above them, i.e., they will gen- 

 erally be parallel to the coast-line. Hence the continents will 

 grow by the formation of mountain chains along their borders. 



" In a given time, the amount of rock-folding resulting from 

 secular cooling was greatest in the first epochs of the earth's his- 



