Parr IL. Srcr. iii.§3.] ANIMAL FORMATIONS. — 468 
vegetable life is of primeimportance. In marshy flats where stagnant 
water receives a supply of the organic acids from decomposing plants 
the salts of iron are attacked and dissolved. Exposure to the air 
leads to the oxidation of these solutions and the consequent precipita- 
tion of the iron in the form of hydrated ferric oxide, which, mixed 
with similar combinations of manganese, and also with silica, phos- 
phoric acid, lime, alumina and magnesia, constitutes the bog-ore so 
abundant on the lowlands of North Germany and other marshy tracts 
of northern Europe.t. On the eastern sea-board of the United States 
large tracts of salt marsh, lying behind sand-dunes and bars, form 
receptacles for much active chemical solution and deposit. There, 
as in the 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 the sea-water and 
of organic matter, the iron is there partially reduced into sulphide.” 
The existence of beds of iron-ore among geological formations 
affords strong presumption of the existence of contemporaneous 
organic life by which the iron was dissolved and precipitated. 
The humus acids, which possess the power of dissolving silica, 
precipitate it in incrustations and concretions. Julien describes 
hyalite crusts at the Palisades of the Hudson, due as he thinks, to 
the action of the rich humus upon the fallen débris of diabase. The 
frequent occurrence of nodules of flint and chert in association with 
organic remains, the common silicification of fossil wood, and similar 
close relations between silica and organic remains, point to the action 
of organic acids in the precipitation of this mineral. This action may 
consist sometimes in the neutralization, by organic acids, of alkaline 
solutions charged with silica;* sometimes in the solution and rede- 
posit of colloid silica by albuminoid compounds, developed during the 
decomposition of organic matter in deposits through which silica has 
been disseminated, the deposit taking place preferentially round some 
decaying organism or in the hollow left by its removal.‘ 
Animals.—Animal formations are chiefly composed of the re- 
mains of the lower grades of the animal kingdom, especially of 
Mollusca, Actinozoa, and Foraminifera. 
(1.) Calcareous—Lime, chiefly in the form of carbonate, is 
the mineral substance of which the solid parts of invertebrate 
animals are mainly built up. Hence the great majority of the 
accumulations formed of animal remains are calcareous. In fresh 
water they are represented by the marl of .lakes—a white, chalky 
deposit consisting of the mouldering remains of Mollusca, Entomos- 
traca,and partly of fresh-water alez. On the sea-bottom, in shallow 
water, they consist of beds of shells, as in oyster-banks. Here and 
1 Forchhammer, Neues Jahrb. 1841, p. 17. 
? Julien, Amer. Assoc. 1879, p. 347. 
3 Leconte, Amer. Journ. Sct. 1880, p. 181. 
* Julien, op. cit. 396. Sollas, Ann. Mag. Nat. Hist. Nov. Dec. 1880. 
