SKLECTIVE ABSORPTION P.Y THE EARTH's CRUST. 183 



tion of the chert under pressure, as quartz. The great time neces- 

 sary for a thick deposit to form in this way, in contrast to the 

 comparatively short time requisite in later periods, is due to the 

 fact that in those ages the silica was spread out over the ocean 

 floor without reference to the boundaries, whereas the heavier 

 mechanical deposits of later times were unable to be carried far 

 from the land, and were hence deposited within at least two 

 hundred miles of the shore. When sufficient material had ac- 

 cumulated, and earth movements had elevated the recrystallised 

 siliceous sediments above the sea-level, then, for the first time, 

 quartz became subjected to weathering, and the first true sand 

 grains began their travel. 



Alumina, united with silica and water as kaolin and clay, 

 was formed from the commencement of the action of water and 

 atmospheric oxygen on the rocks of the earth's crust exposed 

 above sea-level. 



The iron at once became absorbed, and travelled down- 

 wards as it does to-day. Where it goes to, and why. is a mys- 

 tery I have tried to explain in the following way. First, as to 

 why it goes downwards. W^eak solutions of salts become ionised ; 

 that is to say, become split up into two parts ; a metallic part 

 which carries a positive charge of electricity, and an acid part 

 which carries a negative charge. The positive, or metallic ion, 

 is in the elemental condition, and hence, if a compound of iron 

 is dissolved, a portion of it exists in the solution as metallic iron, 

 and, as such, should be aft'ected by the magnetic attraction of the 

 earth's core. 



Where it goes is a matter which we can, to a certain extent, 

 explain by noticing that under special conditions deep-seated beds. 

 of limestone may become wholly converted into haematite, or 

 iron oxide, by the percolation through them of solutions of iron 

 derived from above. Generally speaking, also, the older the beds 

 the more highly ferruginous they are ; that is to say, when they 

 were buried, the continual passage of iron-solutions through the 

 interstices betwen the grains caused a certain amount of deposi- 

 tion of iron within them, bnder certain circumstances the de- 

 scending water may be returned through fissures to the surface 

 as chalybeate springs, but the vast preponderance of iron dis- 

 solved from the rocks, as they weather on the surface of the earth, 

 passes directly downwards and becomes lost, as far as our actual 

 knowledge goes. It must accumulate at the base of the crust, 

 beyond which no water derived from the surface can descend. 

 Practically, no iron in solution reaches the sea in river water, 

 however much iron-bearing rocks there may be exposed on the 

 continent drained by the rivers. Where drainage is deficient, as 

 on the flat coastal plateaux in South Africa, the iron in solutiou 

 is precipitated under the soil, probably by organic agency, cer- 

 tainly the iron-bacterium Crcnothrix is present in soil, but 

 whether this is entirely responsible for the deposition, or whether 

 *;ome chemical deposition is superadded, is not yet determined. 



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