Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/397

FORAMINIFERA 383 larger Foraminiferal types last described, on the other hand, continuous gemmation proceeds much further ; and thus we may have in an Orlitolite, a Nummulite, or a C ycloclypeits, hundreds or even thousands of chambers, all formed by the segmental extension of tho animal body from one " primordial segment." Further, in Polytrema we find a rotaline shell extending itself by the continuous gemma tion of the animal, without any definite limit either of size or shape, its increase taking place by vertical as well as by horizontal gemmation, so that it develops itself into forms which are essentially zoophytic. Hence there is nothing in the nature of Foraminiferal organization in the least degree inconsistent with the idea that a shell essentially "Nummuline" in its character should extend itself indefinitely by the continuous gemmation of the animal that forms it, not merely over horizontal planes, but also in the vertical direction, so as to produce solid massive struc tures bearing a strong general resemblance to Coral growths. Again, the dead shells of Foraminifera brought up from many parts of the sea-bottom are found to have their cavitierf filled with green or ochreous ferruginous silicates, which seem allied in composition to the " glauconite " of the Greensand formation; and these mineral deposits occupy not merely the chambers of tho shell, but its canal- system alao ; so that when the calcareous shells have been dissolved away by dilute acid (which does not act on their contents), internal casts are obtained, which are perfect models, not only of the soft sarcodic segmented body of the animal, but also of its extensions into the canal-system that traverses the substance of the intermediate skeleton (figs. 21, 27). Yet further, this mineral deposit sometimes penetrates the minute tubuli of the shell itself ; so that the surface of the internal cast of each chamber is beset with little points which represent the pseudopodia that ex tended themselves into those tubuli from each sarcodic .segment. There can be no reasonable doubt that these " internal casts " are formed by a process of chemical " sub stitution," the animal body, in the progress of its decay, being replaced, particle by particle, by ferruginous silicates precipitated from sea-water by the liberation of ammonia. And the explanation of the fact that this replacement only occurs in particular localities (notably in the /Egean, over the Agulhas bank near the Cape of Good Hope, and on certain parts of the Australian coast) probably lies in some local peculiarity in the composition of the sea-water, which may not improbably bo due to the discharge from sub marine springs of water containing an unusual quantity of the materials of the deposit. Now there is ample evidence that a similar process has taken place at various geological periods. For, as was first pointed out by Professor Ehreuberg, the green sands which occur in different formations from the Silurian to the Tertiary epoch (being especially characteristic of the earlier Cretaceous) consist very largely (though not exclusively) of the " internal casts " of Foraminifera,- as is shown by microscopic examination of their grains. Examples of such casts, which can be referred without the least difficulty to known Foraminiferal types, have been already given (figs. 17, 27). Hence there is no inherent improbability in such an occupation of the chamber-cavities, of the canal- system, and even of the minute tubulation, of a still earlier Foraminiferal structure, formed by the indefinite extension of a Nummuline growth into coral-like masses. And as there is every reason to believe that the composition of ocean-water has varied greatly at different geological periods, and especially that it contained magnesian salts at earlier epochs in much larger quantity than at present, it might be expected that the silicates which replace tho soft tissues of the animals of the earliest organisms should be essentially magnesian. This expectation has been singu larly confirmed by the fact ascertained by Dr Sterry Hunt, that in certain Silurian Limestones from Wales and New Brunswick, the segments of Crinoidal stems have the soft animal tissue, which originally filled the interspaces of their calcareous network, replaced by a silicate of alumina, iron, magnesia, and potass intermediate between glauconite and serpentine. Now, the Serpentine-limestone that lies near the base of the Laurentian formation in Canada is ordinarily composed of alternating layers of carbonate of lime (in the condition of calcite) and of silicate of magnesia (serpentine), these layers being superposed upon one another with great general regularity (as shown in fig. 33), and frequently exceeding fifty of each kind in number. In some localities, however, the calcite is replaced by dolomite (magnesian limestone), while the serpentine is replaced either by loganite (a compound of silex, alumina, magnesia, and iron), or by some other magnesian silicate; but the alter- fir.. 33. Section of the Eozoic Serpentine-limestone of Canada, snowing alter nation of calcareous (light) and serpent inous (doik) lamella!. nation of calcareous and siliceous lamellae is just as regular in these cases as in the typical ophicalcite, and there is every probability that the dolomitic layers were originally formed as calcite, and subsequently metamorphosed by magnesian infiltration. The regularity of this alternation of calcareous and siliceous lamellas, which has no parallel in any undoubted mineral, taken in connexion with the re semblance of the entire formation (extending continuously over hundreds of square miles) to ancient coral reefs, hav ing suggested to Sir William Logan its organic origin, a careful microscopic examination of specimens that seemed to have been least altered by metamorphic action was made at his instance by Dr Dawson of Montreal, with results that left him in no doubt either of the organic nature of the calcareous lamellae, or of the Foraminiferal affinities of the organism. These results were confirmed by the writer of this article, whose re-examination of the subject, to meet the objections raised from time to time against Dr Dawson s conclusions, has only had the effect of strengthening his original conviction of their truth. And the following general sketch of the structure of Eozoon canadense is here presented with full confidence, as expressing, not merely his own views, but those of the numerous eminent naturalists who have examined for themselves the evidentiary facts placed before them. 1 1 Of those who may be considered as qualified by special study of the Foraminifera to form a judgment on this matter, Mr H. Carter is the only one who has expressed his dissent ; and he has never seen