HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 107 



of life. The evolution of the ocean basins and the continental plat- 

 forms under this hypothesis is, however, exceedingly simple. 



The evolution of oceanic basins and continental platforms. 



With the acquisition of an atmosphere and hydrosphere, the con- 

 ditions for weathering were present, and all those attendant processes 

 of a gradational nature which constitute the dominant surface work 

 of to-day, and which have been sketched at length in Volume I, were 

 inaugurated. 



Contrasted effects of weathering and protection by submergence. — 

 For the present study, two features of these gradational processes over- 

 shadow all others: (1) the weathering and leaching action on the land, and 

 (2) the relative protection under the water-bodies. The chemical changes 

 wrought by the weathering of the surface of the land have been set 

 forth in Volume I (pp. 41-43, 422, 426-430, 474-485). The essence 

 is this : through the action of the atmosphere and atmospheric waters, 

 the basic material is more largely dissolved and carried away than 

 the acidic. When the weathering is thorough, the residue is chiefly 

 quartzose sand — if the original rock contained quartz — and various 

 residual earths and clays which are essentially silicious silts and 

 aluminum silicates, with a low percentage of the basic oxides. If these 

 earths and clays are turned back into crystalline rocks by metamorphism, 

 they form acidic schists or gneisses, while the quartzose sand becomes 

 quartzite. The material borne away in solution consists mainly of 

 compounds of the alkalies and alkaline earths. A part of this is rede- 

 posited within the zone of the hydrosphere beneath the land, and a 

 part is borne to the sea and remains in solution, or is deposited beneath 

 it. Although some decomposition takes place in the zone of the hydro- 

 sphere beneath the land, and some also beneath the permanent water- 

 bodies, it is clearly less than that which takes place in the zone of the 

 atmosphere, and this difference in the sum total of work done, is all 

 that need here be considered. There can be no question that the land 

 areas lose by leaching, and the water areas gain correspondingly. The 

 general effect is an increase in the acidity and a reduction of the specific 

 gravity of the land material. This includes the land wash deposited 

 on the borders of the continents. 



Areal differentiation by selective action. — Now when the growing 

 hydrosphere crept up to the surface and covered the lower tracts. 



