l82 SELECTIVE ABSORPTION I5V THE EARTH 's CRUST. 



pan in the form of gypsum and common salt, the former ahiiost 

 invariably crystallising, either in the mud below the surface, or 

 just on the surface, while the salt remains obstinately on the 

 top of all. We should expect, from the analysis of the dolerite, 

 that there would be a mucn greater deposit of iron and, to a 

 lesser extent, of magnesia, but of the former there is no trace 

 whatever, while the latter may be intercepted in wells and borings 

 round the edge of the pan, where it makes the water bitter from 

 the epsom salts dissolved in it. W'e have here, then, a clear 

 instance of the selective absorption of the earth's crust, which 

 acts like a sieve to iron, causing it to sink immediately down- 

 wards, letting the magnesium through, though more slowly than 

 iron, but forming an absolutely impervious medium to the lime 

 and soda salts. 



If the whole country were composed of dolerite, and this ac- 

 tion were to continue for a very long time, till a very thick layer 

 were completely weathered, we should find that that portion of the 

 earth's surface would eventually consist of silica, alumina, lime 

 and soda salts. In due time some of the silica would be con- 

 verted into quartz, which would form the grains of sandstones ; 

 some of it would remain combined with alumina, as kaolin, in 

 the form of clay; the lime would unite with carbon dioxide of 

 the air (the sulphur of the gypsum supplied by the pyrites in 

 the shales of the normal South African kopjes would be absent), 

 and the soda would be represented by salt. That is to say, the 

 essential constituents of the normal three types of sedimentary 

 rocks, sandstones, shales and limestones, would be present, and 

 the salt of the ocean would also be there ; the iron and magnesium, 

 so rich in the original rock, would be represented by small traces. 

 If the whole earth were originally made of rocks of the dolerite 

 composition, then the sediments would form in the manner they 

 do now, and with the same composition. On Chamberlin's 

 hypothesis, the earth is supposed to have originally consisted of 

 ultra-basic rocks and iron. The composition of the ultra-basic 

 rocks is practically the same as that of the dolerite, with less 

 silica, hence the selective absorption of the earth's crust, directly 

 water became formed, would result in the crust as we find it, 

 with silica, alumina, lime and soda predominating over all other 

 oxides of elements — disregarding hydrogen oxide, or water and 

 oxygen of the air, which form the media by which the absorption 

 is rendered possible. 



The silica in the earlier periods would be colloidal silica ; 

 indeed, quartz could not have existed in the earlier stages of the 

 earth's history on the planetcsimal hypothesis, as the continual 

 bombardment of meteorites on the growing sphere would have 

 kept the surface at a white heat, and quartz cannot exist above 

 a temperature of 800 degrees C. The colloidal silica weathered 

 from the basic silicates would be carried to the ocean, and would 

 there form layers of chert. After immense periods of deposition, 

 sufficient sediment would accumulate to allow the recrystallisa- 



