Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/142

126 PACIFIC OCEAN centimetres in thickness (fig. 6). In the same dredgings in the red-clay areas some sharks teeth and Cetacean ear-bones, some of which belong to extinct species, are surrounded with thick layers of the manganese, and others with merely a slight coating. Cosmic The cosmic spherules incidentally referred to under the dcscrip- sphe- tioii of red clay may be here described in greater detail. If a roles, magnet be plunged into an oceanic deposit, especially a red clay from the central parts of the Pacific, particles are extracted, some of which are magnetite from volcanic rocks, to which vitreous matters are often attached ; others again are quite isolated, and differ in most of their properties from the former. The latter are generally round, measuring hardly 2 mm., usually smaller; their surface is quite covered with a brilliant black coating having all the properties of magnetic oxide of iron ; often there may be noticed clearly marked upon them cup-like depressions (figs. 7 and 8). If these spherules be broken down in an agate mortar, the brilliant black coating easily falls away and reveals white or grey metallic malleable nuclei, which may be beaten out by the pestle into thin lamellae. This metallic centre, when treated with an acid solu tion of sulphate of copper, immediately assumes a coppery coat, thus showing that it is native iron. But there are some mal leable metallic nuclei extracted from the spherules which do not give this reaction ; they do not take the copper coating. i. Fig. 8. FIG. 7. Black Spherule with Metallic Nucleus (x60). This spherule covered with a coating of black shinii]g~magnetite represents the most frequent shape. The depression here shown is often found at the surface of these spherule*. From 2375 fathoms, South Pacific. Fio. 8. Black Spherule with Metallic Nucleus (x60). The black external coat ing of magnetic oxide has been broken away to show the metallic nucleus re presented by the clear part at the centre. From 3150 fathoms. Chemical reactions show that they contain cobalt and nickel ; very probably they constitute an alloy of iron and these two metals, such as is often found in meteorites, and whose presence in large quantities hinders the production of the coppery coating on the iron. G. Rose has shown that this coating of black oxide of iron is found on the periphery of meteorites of native iron, and its presence is readily understood when their cosmic origin is admitted. Indeed, these meteoric particles of native iron in their transit through the air must undergo combustion, and, like small portions of iron from a smith s anvil, be transformed either entirely or at the surface only into magnetic oxide, and in the latter case the nucleus is protected from further oxidation by the coating which thus covers it. One may suppose that meteorites in their passage through the atmosphere break into numerous fragments, that incandescent particles of iron are thrown off all round them, and that these eventually fall to the surface of the globe as almost impalpable dust, in the form of magnetic oxide of iron more or less completely fused. The luminous train of falling stars is probably due to the combustion of these innumerable particles resembling the sparks which fly from a ribbon of iron burnt in oxygen, or the particles of the same metal thrown off when striking a flint. It is easy to show that these particles in burning take a spherical form, and arc surrounded by a layer of black magnetic oxide. Among the magnetic grains found under the same conditions as those just described are other spherules, which are referred to the chondres, so that, if the interpretation of a cosmic origin for the magnetic spherules with a metallic centre were not established in a manner absolutely beyond question, it almost becomes so when their association with the silicate spherules, which will now be described, is taken into account. It will be seen by the microscopic details that these spherules have quite the constitution and structure of chondres so frequent in meteorites of the most ordinary type, and on the other hand they have never been found, as far as is known, in rocks of a terrestrial origin ; in short, the presence of these spherules in the deep-sea deposits, and their association with the metallic spherules, are matters of prime importance. Among the fragments attracted by the magnet in deep-sea deposits are distinguished granules slightly larger than the spherules with the shining black coating above described. These are yellowish-brown, with a bronze-like lustre, and under the microscope it is noticed that the surface, instead of being quite smooth, is grooved by thin lamellae. Their dimensions never attain a millimetre, generally they are about - 5 mm. in diameter ; they are never perfect spheres, as in the case of the black spherules with a metallic centre ; and sometimes a depression more or less marked is to be observed in the periphery. "V hen examined by the micro scope it is observed that the lamella; which compose them arc applied the one against the other, and have a radial eccentric dis position. It is the leafy radial structure (radialblattrig), like that of the chondres of bronzite, which predominates in these spherules. The serial structure of the chondres with olivine is observed much less rarely, and indeed there is some doubt about the indications of this last type of structure. Fig. 9 shows the characters and texture of one of these spherules magnified 25 diameters. On account of their small dimensions, as well as of their friability due to their lamellar structure, it is difficult to polish one of these spherules, and it has been necessary to study them with reflected light, or to limit the observations to the study of the broken fragments. These spherules break up following the lamella?, which latter are seen to be extremely fine and perfectly transparent. In rotating between crossed nicols they have the extinctions of the rhombic system, and in making use of the condenser it is seen that they have one optic axis. It is observed also that when several of these lamella; are attached they extinguish exactly at the same time, so that everything tends to show that they form a single individual. In studying these trans parent and very thin frag ments with the aid of a high magnifying power, it is ob served that they are dotted with brown-black inclu- FlG . 9. Spherule, of Bronzite (x 25), from 3500 sions, disposed with a ccr- fathoms in the Central South Pacific, show- tain symmetry, and showing in S many of the peculiarities belonging to i . i chondres of bronzite or enstntitc. somewhat regular contours ; these inclusions are referred to magnetic iron, and their presence explains why these spherules of bronzite are extracted by the magnet. It should be observed, however, that they are not so strongly magnetic as those with a metallic nucleus. They are designated bronzite rather than cnstatite, because of the somewhat deep tint which they present ; they are insoluble in hydrochloric acid. Owing to the small quantity of substance, only a qualitative analysis could be made, which showed the presence in them of silica, magnesia, and iron. The study of deep-sea deposits suggests some interesting conclu- Great sions. It has been said that the debris carried away from the land anti- accumulates at the bottom of the sea before reaching the abysmal quity of regions of the ocean. It is only in exceptional cases that the finest oceanic terrigenous materials are transported several hundred miles from areas, the shores. In place of layers formed of pebbles and clastic elements with grains of considerable dimensions, which play so large a part in the composition of emerged lands, the great areas of the ocean basins are covered by the microscopic remains of pelagic organisms, or by the deposits coming from the alteration of volcanic products. The distinctive elements that appear in the river and coast sediments are, properly speaking, wanting in the great depths far distant from the coasts. To such a degree is this the case that in a great number of soundings, from the centre of the Pacific for example, no mineral particles on which the mechanical action of water had left its imprint have been distinguished, and quartz is so rare that it may be said to be absent. It is sufficient to indicate these facts in order to make apparent the profound differences which separate the deposits of the abysmal areas of the ocean basins from the series of rocks in the geological formations. As regards the vast deposits of red clay, with its manganese concretions, its zeolites, cosmic dust, and remains of vertebrates, and the organic oozes which are spread out over the bed of the Central Pacific, Atlantic, and Indian Oceans, have they their analogues in the geological series of rocks? If it be proved that in the sedimentary strata the true pelagic sediments are not represented, it follows that deep and extended oceans like those of the present day cannot formerly have occupied the areas of the present continents, and as a corollary the great lines of the oceanic basins and continents must have been marked out from the earliest geological ages. Without asserting that the terrestrial areas and the areas covered by the waters of the great ocean basins have had their main lines marked out since the coimnencement of geological history, it is a fact proved by the evidence of the pelagic sediments that these areas have a great antiquity. The accumulation of sharks teeth, of the ear- bones of Cetaceans, of manganese concretions, of zeolites, of volcanic material in an advanced state of decomposition, and of cosmic dust, at points far removed from the continents, tends to prove this. There is no reason for supposing that the parts of the ocean where