Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/399

 FORAMINIFERA 385 being especially the case with the smaller and simpler re presentatives of them that range over those vast areas in the deep oceanic basins, of which the bottom-temperature is kept down by the polar underflow. A striking resem blance has long been noted between the poor and feeble Foraminiferal fauna of shallow waters in colder-temperate seas, and that of great depths in tropical seas ; and this similarity is now explained by the correspondence of the temperature to which these two faunae are subjected. The larger and most developed examples of existing Fora minifera, on the other hand, are limited to the shallower seas near tropical shores, or covering coral reefs where the bottom-temperature is comparatively high ; or to partially enclosed seas, like the Mediterranean and lied Sea, which are cut olf from the polar underflow by the shallowness of the straits which connect them with the oceanic basin. It is in such seas that we meet with the largest Orbimlince, and the most complex Orbitolites and Alveolince, the first of these &quot; imperforate &quot; types being generally abundant in tropical shore-sands, the second in shallow dredgings along the Australian coast, the great barrier-reef, and the lagoons of &quot; atolls &quot; in the Coral Sea, while the third seems to attain its highest development in the Philippine Seas. So, again, among the &quot; perforate &quot; Lagenida, we find the &quot; nodosarian &quot; and &quot; cristellarian &quot; types attaining a very high development in the Mediterranean ; the most complex forms of the &quot; rotalian &quot; type, including the zoophytic Polytrema, are only met with in tropical or sub-tropical seas ; while, with the exception of small and modified Folystomellai and dwarfed Operculince, there are no known representatives in the colder-temperate or polar seas either of the gigantic Cycloclypeus, or of the large Operculina and Heterosteyina of the tropics. But while the Foraminiferal fauna thus obviously depends on an elevated temperature for the attainment of its highest development as regards the size and complexity of its individual members, the numerical multiplication of its lower forms (as is the case in many other groups) seems to be favoured by a much lower degree of warmth ; so that we find the vast area of &quot; globigerina-ooze &quot; extending to the borders of the Polar seas. There, however, it ceases almost abruptly, the place of this calcareous deposit being taken by an accumulation of the siliceous skeletons of JRadiolarians and Diatomaceae. It is not a little curious, however, that recent researches should have disclosed the fact, that the existing Arenace ous Lituolida attain their highest development in regard no less to size and complexity of structure than to number on the deep sea-bottom, the additions to our previous knowledge of the Foraminiferal fauna made by the dredg ings of the &quot; Porcupine,&quot; the &quot; Valorous,&quot; and the &quot;Chal lenger &quot; having been far greater in this section than in either of the calcareous-shelled groups. And it would hence appear that an elevated temperature is not as essential to the high development of the Foraminifera which construct &quot; tests &quot; by gluing together grains of sand, as it .is to that of the shell-forming types which separate the material of their &quot; porcellanous &quot; or &quot; vitreous&quot; skeletons from the sea- water in which they live. Geological Distribution. There is no division of the Animal Kingdom whose range in time (so far as is at present known) 1 can be compared with that of the Foraminifera. Looking, indeed, to the vast series of ages that must have been required for the deposit of that long succession of Upper Laurentian and Huronian rocks which intervenes between the Eozoic Limestone of the Lower Laurentians of Canada, and the lowest strata in which the most ancient representatives of the Palaeozoic fauna have ag yet been 1 Appearances resembling Annelid burrows have been found in Laurentian rocks ; but these cannot be safely relied on as evidence of the existence of Marine Worms at that epoch. found, it may even be said that all other fossils are modern by comparison. For the interval between the formation of the Canadian Eozoon and the period represented by the oldest fossils of the Lower Cambrian series seems undoubtedly to have been quite as great geologically speaking as that which intervened between the latter and the existing epoch, if not greater, the &quot; fundamental gneiss &quot; of Sir Roderick Murchison, which represents in cen tral Europe the Laurentians of Canada, and near the base of which is found the kindred Eozoon bavaricum, having a thickness estimated at 90,000 feet, and being overlaid by a great thickness of other non-fossiliferous rocks. Hence the determination of the organic origin of this Ophicalcite, and of its Foraminiferal affinities, which has been effected by the examination and comparison of parts of specimens so minute as to be scarcely visible to the naked eye, must be considered as one of the most remarkable results of micro scopic research fully equal in importance, when considered in all its bearings, to the discovery by Prof. Ehrenberg of the Foraminiferal origin of Chalk.- Indications of Eozoic structure have been found in various strata of uncertain age underlying the Silurians of North America, and also in some of the older rocks of the Scan dinavian series ; and there is no improbability in the idea that its existence may have been prolonged through the whole of that long period, commonly regarded as azoic, which has been designated by Prof. Dana as Archaean. In the earliest strata usually accounted fossiliferous there are found, alike in the old and new continents, a number of curious organisms, sometimes of considerable size, which have received the n&mes Archceocyathus, Stromatopora, and Receptacidites. The nature of these is still problematical, their internal structure not having yet been fully eluci dated ; but while it seems probable, on the one hand, that among the organisms whose similarity of external confor mation has led to their association under these names, there may be several types of structure essentially different, there seems considerable reason to believe that some among them are really gigantic Foraminifera, presenting approximations to calcareous Sponges. The Limestones of the Silurian period have not been as yet minutely searched for the smaller forms of Foraminifera ; but green sands of Silu rian age occur in various localities, the grains of which can be identified as &quot; internal casts&quot; of Foraminiferal shells. It is in the Carboniferous Limestone that we first recognize a varied and abundant Foraminiferal fauna, which has recently been made the subject of special study by Mr H. B. Brady, 3 whose very interesting results may be summed up as follows. (1) Of the &quot;imperforate&quot; or &quot;porcel lanous &quot; Foraminifera no examples have been detected. (2) Of the &quot; perforate &quot; or &quot; vitreous &quot; series, on the other hand, each principal group is represented : the Lagenida very slightly ; the Globigerinida by Textularia and various Rotalian types ; while of the Nummulinida we find, not only three generic forms that are at the same time indivi dually small and scantily diffused, but (in certain localities) such an accumulation of comparatively large shells of FusuUna (fig. 20), that they constitute almost the sole material of calcareous beds extending over large areas. (3) A large proportion of the foraminiferal types of this period belong to that &quot; arenaceous &quot; group which at present contains not merely the Lituolida, whose &quot; tests &quot; are entirely made up of cemented sand-grains (among which 2 &quot; The discovery of organic remains in the crystalline limestones of the ancient gneiss of Canada,&quot; says Prof. Giimbel, the accomplished director of the Geological Survey of Bavaria, &quot;for which we are indebted to the researches of Sir William Logan and his colleagues, and to the careful microscopic investigations of Drs Dawson and Carpenter, must be regarded as opening a new era in geological science.&quot; 3 Monograph of the Carboniferous and Permian Foraminifera, published by the Palseontographical Society, 1876. IX. --49