Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/400

386 FORAMINIFERA the Saccamina, fig. 5, a, b, is distinctly recognizable), but also a number of generic types more or less allied to the recent Valvulina, iu which there is a " vitreous " shelly basis, more or less thickly incrusted by an arenaceous envelope. The " isomorphism" of these with true " nodo- sarines " and " rotalines " suggests the question whether they are not really in an evolutionary stage between the true sandy-tested and the completely calcareous-shelled types. But there is a larger question even than this, namely, whether those massive beds of Carboniferous Limestone whose texture is sub-crystalline, and in which scarcely any traces of organic structure are now discernible, do not owe their origin, like analogous beds of the Cretaceous forma tion, to Foraminiferal life. The general tendency of recent geological opinion has been to account for the absence of all traces of organic structure in such beds by the " meta- morplmm" they have undergone subsequently to their original deposition. Of the completeness with which such metamorphism may obliterate the evidence of organic origin in calcareous rocks it is impossible to have more " pregnant instances " than those afforded by the purely crystalline marble of Carrara, which is unquestionably a metamorphic Oolite, and the crystalline conversion of the Antrim Chalk in the neighbourhood of the basaltic out burst which forms the Giant s Causeway. The abundance of Corals, however, in various beds of Carboniferous Lime stone, accompanied by such accumulations of Brachiopods, Crinoids, &c., as might have been associated with them on coral reefs, taken in connexion with the fact that the substance of existing reef-building corals, when raised by upheaval, sometimes exhibits a metamorphism that causes it to bear a close resemblance to carboniferous limestone, has seemed to favour the belisf that the azoic sub-crystalline beds of that formation had their origin in Coral growths. Against this, however, it may be urged that we have at the present time no continuity of Coral-growth over areas that are at all comparable to those which we find continuously covered with Carboniferous Limestone ; that the real parallel to these is presented by the continuous beds of Chalk formerly deposited over vast marine areas, by those of the Nummuli- tic Limestone of the succeeding period, and by those of Globigerina-ooze still in progress of production ; and that, if the tropical sea-bottom were now elevated into dry land, we should find over the deeper areas a continuous Forami niferal deposit, interrupted in particular localities by upheaved reefs of Coral, whose slopes would be covered with calcareous-shelled Mollusks, Echinoderms, &c., bearing a general correspondence with the great Carboniferous Lime stone formation. Moreover, it has now come to be generally admitted that "metamorphism" is connected with mechanical disturbance, and especially with that " lateral thrust" which is the prin cipal source of the plication and contortion of strata, and which must have given rise to the evolution of great heat. That such a change was extensively produced in the Car boniferous rocks of Great Britain at the end of the Paleo zoic period, when the horizontal and continuous Coal- measures were thus broken up into separate "basins," bordered by intervening ridges formed by the uplifting of their subjacent limestone, is universally recognized. But, on the other hand, over the vast area of Eussia that lies between the Baltic and the Ural mountains, there seems to have been no such force in action. Its strata are so nearly level that the determination of their order of superposition lias been difficult. And there is such a remarkable absence of metamorphism even in those of Silurian age that we there find what are elsewhere slates represented by beds of clay, constituting the original form of that deposit which has elsewhere acquired the slaty character by subsequent metamorphism. Now, since it is here that we also find the most extensive development of ^ww/ma-limestone in the place of those sub-crystalline beds which elsewhere form the great bulk of the Carboniferous Limestone, there seems exactly the same reason for regarding such a Fora miniferal deposit as the original form of those beds. And this view derives remarkable confirmation from the pheno mena presented by the Carboniferous Limestone of Ireland, which has been recently studied with great care by 1 rof. Hull. 1 This formation extends over a very considerable area of the island, constituting in its central portion an elevated plateau, whose nearly horizontal strata present no indication of lateral thrust, whilst along the margin of this area the calcareous strata have been elevated into ridges by thrust from the outside, with more or less of plication and contortion. Now, in these marginal ridges the traces of organic origin are scanty, the presence of fossils, whether large or minute, being the exception and not the rule. In the level strata of the central plateau, on the other hand, the evidence of organic origin is almost everywhere complete; and whilst fossil Corals, Mollusks, Crinoids, Arc,, are abundant in particular beds, great masses of the lime stone are found by microscopic examination to consist of little else than minute Foraminiferal shells. A very strong case, then, seems made out for the belief that Foraminiferal life has contributed very largely to the production of those vast beds of Carboniferous Limestone (often exceeding 1500 feet in thickness) in which scarcely any trace of organic origin can now be found, this share bearing to that of the Corals, which are generally credited with the whole, some what of the same proportion that the Foraminiferal ooze of the deep-sea-bottom at the present time does to the Coral reefs of shallow waters, and the shells of Mollusks and Echinidans that live among them. In the earlier Secondary rocks we do not find by any means the same evidence of abundant Foraminiferal life ; but this is very probably to be accounted for by the condi tions of their deposition. For during the Triassic period we find no representative of a deep oceanic calcareous deposit, the only considerable limestone bed (the muschel- kalk) being obviously composed of the remains of the more developed fauna of comparatively shallow water. It is interesting, however, to find certain clay beds of the New Red Sandstone yielding Foraminifera, chiefly of the Cristel- larian type, which can be identified not only generically and specifically, but even varietally with forms common in the Italian Tertiaries, and still living in the Mediter ranean. In many of the Liassic clays Foraminifera are found iu great abundance ; and here the " porcellanous " group first characteristically shows itself under one of its least modified types, the Spiroloculina. In the formation of many Oolitic Limestones it seems probable that Forami nifera had a considerable share. For while the material of many of the calcareous beds belonging to this formation was obviously furnished by debris of Corals, Echinoderms, and Mollusks, the nuclei of the component grains of the true Oolitic beds are so often found to be minute Foramini fera, that we seem justified in regarding those beds as having been formed by a sub-aerial metamorphosis of shore sands largely composed of the minute shells belonging to that group, like many tropical shore-sands of the present time. It was by the comparative study of specimens of Chalk brought from different localities that Professor Ehrenberg was first led to the conclusion 2 that this vast formation was mainly produced by the progressive accumulation and subsequent disintegration of the minute shells of various 1 Physical Geography and Geology of Ireland, 1878. 2 Ueber die Bildung der Kreidefelsen und des Kreidemergels durch unsichtbare Organismen," Berlin, 1838.