16 Causes and Course of Organic Evolution 



when in 1 per cent, aqueous solution. This solution is coag- 

 ulated at ordinary temperature, by sulphuric acid, alkalies, 

 alkaline carbonates, sulphates, and neutral salts in general. 

 It then is a deep red jelly resembling a clot of blood, but more 

 transparent. 



The extensive prevalence of such colloid iron oxides in waters 

 of archsean age seems exactly to explain the origin and abundant 

 deposits of bog iron ore, of haematite, and of magnetic iron. 

 At this time, as will be emphasized later, a simple but abundant 

 vegetable and animal life must have been evolving. Thus 

 Geikie (IS: 612) says: "In marshy flats and shallow lakes, 

 where the organic acids are abundantly suppUed by decom- 

 posing plants, the salts of iron are attacked and dissolved. 

 Exposure to the air leads to oxidation of these solutions, and 

 consequent precipitation of the iron in the form of hydrated 

 ferric oxide, which, mixed T^ath similar combinations of manga- 

 nese, and also with silica, phosphoric acid, lime, alumina, and 

 magnesia, constitutes the bog-ore so abundant in lowlands of 

 North Germany and other marshy tracts of North Europe. 

 On the eastern seaboard of the IJnited States, large tracts 

 of salt marsh, lying behind sand-dunes and bars, form recep- 

 tacles for much active chemical solution and deposits. There, 

 as in European bog-iron districts, ferruginous sands and rocks 

 containing iron are bleached by the solvent action of humus 

 acids, and the iron removed in solution is chiefly oxidized, 

 and thrown down on the bottom. In presence of the sulphates 

 of sea water and of organic matter, the iron of ferruginous 

 minerals is partially changed into sulphide, which on oxidation 

 gives rise to precipitation of bog-iron. The existence of beds 

 of iron ore among sedimentary formations affords strong pre- 

 sumption of the existence of contemporaneous organic life, 

 by which the iron was dissolved and precipitated." 



Compounds of silicon, arsenic, and other nonmetals can 

 also assume the colloid state. Thus Ostwald says (18: 429), 

 "in nature silicic acid occurs very often in such a form. It 

 gets into natural waters from the silicates when these are decom- 

 posed by CO2." It may remain fluid for days or weeks in a 

 sealed tube, but is sure to gelatinize and become insoluble at 

 last. Such colloid silica may be precipitated by the action of 

 constituents of decaying plants or animals, when it may take 

 the form of extensive siliceous deposits over the bed of the 

 ocean, or round the edges of geysers (14), as in the Yellowstone 

 region and New Zealand, amongst many others. 



