184 SELECTIVE ABSORPTION BV THE EARTH's CRUST. 



It is significant, however, that where there is no soil, and, hence, 

 no medium for the Crcnothrix to live in, there no iron becomes 

 arrested on the surface. The superficial deposits of this surface 

 iron are very large in South Africa, forming ironstone gravel, 

 ou-klip, or, as it is known in England, moor-bed stone (ort-stcin 

 in Germany). In cold latitudes the iron in the waters draining 

 into lakes is likewise arrested again by organic agency, by various 

 iron Bacteria, by the diatom GalUoncUa, and even by the leaves 

 of higher plants, such as Elodca; but here, again, it is not certain 

 whether the organisms are solely responsible for the deposition, 

 but the constant presence of phosphorus in iron thus precipitated 

 makes it probable that that is the case.* 



Magnesium follows iion, but here we have a metal which is 

 not magnetic ; nevertheless, all substances are probably magnetic 

 to a certain extent, and hence, from the analogy' of iron, it is 

 conceivable that the reason why magnesium descends in the 

 earth's crust is the same that causes iron to do so, namely, that, 

 as the earth's core consists largely of magnesium, the attraction 

 of this on the ionised magnesium, in the weak solutions derived 

 from weathering rocks, is sufficient to draw it towards the earth's 

 centre. 



That magnesium docs descend we have already seen in the 

 case of the weathering dolerite, but, on a larger scale, we see it in 

 its descent in the dolomitisation of limestones. The older the 

 limestone the more it is dolomitised, generally speaking. That 

 this is a subsequent effect, and is not due to original composition, 

 is proved from the fact that the dolomitisation progresses from 

 the joints inwards towards the centres of the blocks. If a lime- 

 stone, no matter how old it is, lies in a region where earth move- 

 ments have not taken nlace, then the exchange of part of the 

 lime for magnesia does not take place ; but if the same limestone 

 becomes caught in the press of earth movements, then the whole 

 of the limestone is converted into dolomite. There seems to 

 be necessary a certain amount of pressure to render the molecules 

 of the compound sufficiently mobile to allow the exchange to 

 take place. At the same time, this fact proves that a continuous 

 shower of water holding magnesia in solution has gone on in 

 both cases. The feebleness of the attraction of the magnesia 

 centre-wards, however, allows a small percentage of magnesium 

 salts to escape to the ocean, where it accumulates, but in very 

 much smaller proportion than common salt or lime. 



The lime displaced by the magnesia in a limestone which be- 

 comes dolomitised comes to the surface. The same applies to 

 salt, which, if liberated from deposits which have become buried, 

 seeks the surface. These facts are so extraordinary, and I have 

 sought so vainly for any explanation of them, that I may be ex- 

 cused for throwing out a suggestion that may perhaps lead 



* See Molisch, H. ; Sitzungsberichte d. K. Akad d. Wiss., Vienna, 

 October, igio. 



