Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/143

PACIFIC OCEAN 127 these vertebrate remains are found are more frequented by sharks or Cetaceans than other regions where they are never, or only rarely, dredged from the deposits at the bottom. When it is remembered also that these ear-bones, teeth of sharks, and volcanic fragments are sometimes incrusted with two centimetres of manganese oxide, while others have a mere coating, and that some of the bones and teeth belong to extinct species, it may be concluded with great certainty that the clays of these oceanic basins have accumulated with extreme slowness. It is indeed almost beyond question that the red-clay regions of the Central Pacific contain accumulations belonging to geological ages different from our own. The great antiquity of these formations is likewise confirmed in a striking manner by the presence of cosmic fragments, the nature of which has been described. In order to account for the accumulation of all these substances in such relatively great abundance in the areas where they were dredged, it is necessary to suppose the oceanic basins to have remained the same for a vast period of time. The sharks teeth, ear-bones, manganese nodules, altered volcanic fragments, zeolites, and cosmic dust are met with in greatest abundance in the red clays of the Central Pacific, at that point on the earth s surface farthest removed from continental land. They are less abundant in the Radiolarian ooze, are rare in the Globi- gerina, Diatom, and Pteropod oozes, and have been dredged only in a few instances in the terrigenous deposits close to the shore. These substances are present in all the deposits, but owing to the abundance of other matters in the more rapidly forming deposits their presence is masked, and the chance of dredging them is reduced. The greater or less abundance of these materials, which are so characteristic of a true red clay, may be regarded as a measure of the relative rate of accumulation of the marine sediments hi which they lie. The terrigenous deposits accumulate most rapidly ; then follow in order Pteropod ooze, Gloligcrina ooze, Diatom ooze, Radiolarian ooze, and, slowest of all, red clay. From the data now advanced it appears possible to deduce other conclusions important from a geological point of view. In the deposits due essentially to the action of the ocean, the great variety of sediments which may accumulate in regions where the external conditions are almost identical is very striking. Again, marine faunas and floras, at least those of the surface, differ greatly, both with respect to species and the relative abundance of individuals, in different regions of the ocean ; and, as their remains determine the character of the deposit in many instances, it is legitimate to conclude that the occurrence of organisms of a different nature in several beds is not an argument against the synchronism of the layers which contain them. In this connexion may be noted the fact that in certain regions of the deep sea no appreciable forma tion is now taking place. Hence the absence, in the sedimentary series, of a layer representing a definite horizon must not always be interpreted as proof either of the emergence of the bottom of the sea during the corresponding period, or of an ulterior erosion. The small extent occupied by littoral formations, especially those of an arenaceous nature, and the relatively slow rate at which such deposits are formed along a stable coast, are matters of importance. In the present state of things there does not appear to be any thing to account for the enormous thickness of the clastic sediments making up certain geological formations, unless the exceptional cases of erosion which are brought into play when a coast is under going constant elevation or subsidence are considered. Great move ments of the land are doubtless necessary for the formation of thick beds of transported matter like sandstones and conglomerates. Arenaceous formations of great thickness require seas of no great extent and coasts subject to frequent oscillations, which permit the shores to advance and retire. Along these, through all periods of the earth s history, the great marine sedimentary phenomena have taken place. The continental geological formations, when compared with marine deposits of modern seas and oceans, present no analogues to the red clays, Radiolarian, GloMgcrina, Pteropod, and Diatom oozes. On the other hand, the terrigenous deposits of lakes, shallow seas, enclosed seas, and the shores of the continents reveal the equivalents of the chalks, greensands, sandstones, conglomerates, shales, marls, and other sedimentary formations. Such formations as certain Tertiary deposits of Italy and the Radiolarian earth from Barbados, where pelagic conditions are indicated, must be regarded as having been laid down rather along the border of a continent than in a true oceanic area. The white chalk is evidently not a deep-sea deposit, for the Foraminifera and fragments of other organisms of which it is largely composed are similar to those found in comparatively shallow water not far from land. The argillaceous and calcareous rocks recently discovered by Dr Guppy in the upraised coral islands in the Solomon group are identical with the deposits now forming around oceanic islands. Regions situated similarly to enclosed and shallow seas and the borders of the present continents appear to have been, throughout all geological ages, the theatre of the greatest and most remarkable changes ; in short, all, or nearly all, the sedi mentary rocks of the continents would seem to have been built up in areas like those now occupied by the terrigenous deposits. During each era of the earth s history the borders of some lands have sunk beneath the sea and been covered by marine sediments, while in other parts the terrigenous deposits have been elevated into dry land, and have carried with them a record of the organisms which flourished in the sea of the time. In this transitional area there has been throughout a continuity of geological and biological phenomena. From these considerations it will be evident that the character of a deposit is determined much more by distance from the shore of a continent than by actual depth ; and the same would appear to be the case with respect to the fauna spread over the floor of the present oceans. Dredgings near the shores of continents, in depths of 1000, 2000, or 3000 fathoms, are more productive both in species and in dividuals than dredgings at similar depths several hundred miles seawards. Again, among the few species dredged in the abysmal areas farthest removed from land, the majority show archaic characters, or belong to groups which have a wide distribution in time as well as over the floor of the present oceans. Such are the Hcxactincllida, BracMopoda, Stalked Crinoids and other Echino- derms, &c. As already mentioned, the "transitional area" is that which now shows the greatest variety in respect to biological and physical conditions, and in past time it has been subject to the most frequent and the greatest amount of change. The animals now living in this area may be regarded as the greatly modified descendants of those which have lived in similar regions in past geological ages, and some of whose ancestors have been preserved in the sedimentary rocks as fossils. On the other hand, many of the animals dredged in the abysmal regions are most probably also the descendants of animals which lived in the shallower waters of former geological periods, but migrated into deep water to escape the severe struggle for existence which must always have obtained in shallower waters influenced by light, heat, motion, and other favourable conditions. Having found existence possible in the less favourable and deeper water, they may be regarded as having slowly spread themselves over the floor of the ocean, but without undergoing great modifications, owing to the extreme uniformity of the conditions and the absence of competition. Or it may be supposed that, in the depressions which have taken place near coasts, some species have been gradually carried down to deep water, have accommodated themselves to the new conditions, and have gradually migrated to the regions far from land. A few species may thus have migrated to the deep sea during each geological period. In this way the origin and distribution of the deep-sea fauna in the present oceans may in some measure be explained. In like manner, the pelagic fauna and flora of the ocean is most probably derived originally from the shore and shallow water. During each period of the earth s history a few animals and plants have been carried to sea, and have ultimately adopted a pelagic mode of life. ISLANDS. The Pacific Ocean is distinguished from the Atlantic by the greater number of small island groups that diversify its surface. The continental islands, lying along the coasts of America and Asia, have been referred to in speaking of the coasts; the islands of the Malay Archipelago, Australia, New Zealand, and probably New Caledonia belong to the same class. The true oceanic islands on the other hand have no direct geological connexion with the continents; the older sedimentary and metamorphic rocks appear to be quite absent, the islands being either of eruptive or coral formation. The fauna and flora of the oceanic islands present a considerable amount of uniformity, though each island or important group of islands has its peculiar species. There is an entire absence of terrestrial Mammalia. The genera and species are few in number when compared with those of the continents and continental islands from which they would appear to have been originally derived by immigration, and subsequently to have undergone modifica tion. Recent researches appear also to show that the dredgings around oceanic islands yield fewer genera and species than dredgings at similar depths along the shores of continents, although the numbers of individuals of a few species may be extraordinarily abundant. The most northern oceanic group is the Hawaiian Archipelago or Sandwich Islands (see vol. xi. p. 528), stretching for about 340 miles between the latitudes of 18 52 and 22 15 N., and the meridians of 154 42 and 160 33 W. ; it consists of eight large islands Hawaii (Owhyhee), Maui (Mowee), Kahulaui (Tahooroway), Lanai