SELECTIVE ABSORPTION OF SUBSTANCES IN THE 

 EARTH'S CRUST. 



By Professor E. H. L. Sciiwarz, A.R.C.S.. F.G.S. 



On the theory of the earth having passed through a molten 

 stage;, the crust is regarded as a sihceous slag rejected by the 

 cooHng globe which is essentially composed of metals. In this 

 way the specific gravity of the crust, which is about 2.7. is ac- 

 counted for ; the earth, as a whole, is about 5.5, and hence the 

 interior portion, would be about 7, or approximately the specific 

 gravity of iron. Chamberlin's planetesimal hypothesis, and the. 

 developments introduced into the original conception by later 

 authors, account for the earth's centre being of the density ob- 

 served, by supposing that it consists of meteoric matter, essen- 

 tially iron and various basic silicates : the crust, in this assump- 

 tion, would be the result of processes which have taken place on 

 the surface, which have removed the iron and magnesium, and 

 enriched the crust in silica, lime and soda. It is the purpose of 

 this paper to draw attention to the movements of the elements 

 iron, magnesium, calcium, and sodium, at present in action on 

 the earth, which, if not proving the planetesimal hypothesis in 

 all its points, at any rate prove that if the earth had been built 

 up along the lines of that hypothesis, the earth's crust could not 

 have been otherwise than it actually is. 



In South Africa we have a most clear illustration of the 

 movement of the elements named above, in the weathering of 

 dolerite in the Karroo, where no soil hinders the free action of 

 the movement. The ro^k consists of the following substances ; 

 the amounts given are those of a dolerite of the Siebengeliirge 

 on the Rhine, and are typical. 



^\^^^^. 52-63 j practically insoluble. 



Alumuia 13-53 ' 



Iron Oxide . . . . 12.6 



Lime 8.z|4 



Magnesia 6.17 



Potash 1. 61 



Soda . 4.28 



Water 1.55 



100.81 



Such a rock forms kopjes in many parts of South Africa; in 

 many of the drier parts, the water draining from the hillsides 

 is not sufficient to flow away in rivers, but collects at the foot of 

 the hills, in shallow lakes or pans. As there is no outlet for the 

 waters, all substances weathered out of the dolerite, and carried 

 in solution by the water, must come to rest in the pan, and, on 

 evaporation of the water, ought to form a deposit on the floor 

 of the pan. The lime and soda do certainly accumulate in the 



UJ ( L I B R A R Y ) 



