450 



NATURE 



\_AugllSt 2 2, 1878 



closed seas at different temperatures among the islands of the 

 Malay Archipelago ; but we have also a striking instance nearer 

 home. Evaporation is greatly in excess of precipitation over 

 the area T)f the Mediterranean, and consequently, in order to 

 keep up the supply of vcater to the Mediterranean, there is a 

 constant inward current through the Straits of Gibraltar from 

 the Atlantic ; I need not at present refer to an occasional tidal 

 counter-current. The minimum temperature of the Mediter- 

 ranean is about 54° F, from a depth of 100 fathoms to the 

 bottom. The temperature of 54° is reached in the Atlantic at 

 the mouth of the Straits of Gibraltar at a depth of about loo 

 fathoms, so that in all probability future soundings will show 

 that the free water-way through the Straits does not greatly 

 exceed 100 fathoms in depth. 



The Depth of the Sea, and the Nature of Modern Deposits. — 

 It seems now to be thoroughly established by lines of trustworthy 

 soundings which have been run in all directions, that the 

 average depth of the ocean is a little oveq 2,000 fathoms, and 

 that in all probability it nowhere exceeds 5,000 fathoms. Depths 

 beyond 4,000 fathoms are rare and very local, and seem to be 

 usually pits in the neighbourhood of volcanic islands. In all the 

 ocean basins there are depressions extending over considerable 

 areas where the depth reaches 3,000 fathoms or a little more, 

 and these depressions maintain a certain parallelism with the 

 axes of the neighbouring continents. 



Within 300 or 400 miles of the shore, whether in deep or in 

 shallow wa:ter, formations are being laid down, whose materials 

 are derived mainly from the disintegration of shore rocks, and 

 which consequently depend for their stracture and composition 

 upon the nature and composition of the rocks which supply their 

 materials. These deposits imbed the hard parts of the animals 

 living on their area of deposition, and they correspond in every 

 way with sedimentary formations with which we are familiar, 

 of every age. In water of medium depths down to about 2,000 

 fathoms, we have in most seas a deposit of the now well-known 

 globigerina-ooze, formed almost entirely of the shells of forami- 

 nifera living on the sea-surface, and which after death have sunk 

 to the bottom. This formation, which occupies a large part of 

 the bed of the Atlantic and a considerable part of that o f the 

 Pacific and Southern Seas, is very like chalk in most respects, 

 although we are now satisfied that it is being laid down as a rule 

 in deeper water than the chalk of the cretaceous period. 



In depths beyond 2,500 or 3,000 fathoms no such accumula- 

 tions are taking place. The shores of continents are usually too 

 distant to supply land detritus, and although the chalk-building 

 foraminifera are as abundant on the surface as they are elsewhere, 

 not a shell reaches the bottom ; the carbonate of lime is entirely 

 dissolved by the carbonic acid contained in the water during the 

 long descent of the shells from the surface. It therefore be- 

 comes a matter of very great interest to determine what pro- 

 cesses are going on, and what kind of formations are being laid 

 down in these abyssal regions, which must at present occupy an 

 area of not less than ten millions of square miles. 



The tube of the sounding instrument comes up from such 

 abysses filled with an extremely fine reddish clay, in great part 

 amorphous, but containing, when examined under the micro- 

 scope, a quantity of distinctly recognisable particles, organic 

 and inorganic. The organic particles are chiefly siliceous, and 

 for the most part the shells or spines of radiolarians which are 

 living abundantly on the surface of the sea, and apparently in 

 more or less abundance at all depths. The inorganic particles 

 are minute flakes of disintegrated pumice, and small ciystalline 

 fragments of volcanic minerals ; the amorphous residue is pro- 

 bably principally due to the decomposition of volcanic products, 

 and partly to the ultimate inorganic residue of decomposed 

 organisms. There is ample evidence that this abyssal deposit is 

 taking place with extreme slowness. Over its whole area, and 

 more particularly in the deep water of the Pacific, the dredge or 

 trawl brings up in large numbers nodules very irregular in shape, 

 consisting chiefly of peroxide of iron and peroxide of manga- 

 nese, deposited in concentric layers in a matrix of clay, round a 

 nucleus formed of a shark's tooth, or a piece of bone, or an 

 otolith, or a piece of siliceous sponge, or more frequently a 

 fragment of pumice. These nodules are evidently formed in the 

 clay, and the formation of the larger ones and the segregation of 

 their material must have taken a very long time. Many of the 

 sharks' teeth to which I have alluded as forming the nuclei of 

 the nodules, and which are frequently brought up uncoated with 

 foreign matter, belong to species which we have every reason to 

 believe to be extinct. Some teeth of a species of Carcharodon 



are of enormous size, four inches across the base, and are 

 scarcely distinguishable from the huge teeth from the tertiary 

 beds of Malta, It is evident that these semi-fossil teeth, from 

 their being caught up in numbers by the loaded line of the 

 trawl, are covered by only a very thin layer of clay. 



Another element in the red clay has caused great speculation 

 and interest. If a magnet be drawn through a quantity of the 

 fine clay well diffused in water, it will be found to have caught 

 on its surface some very minute magnetic spherules, some appa- 

 rently of metallic iron in a passive state, and some of metallic 

 nickel. From the appearance of these particles, and from th« 

 circumstance that such magnetic dust has been already detected 

 in the sediment of snow-water, my colleague Mr. Murray has 

 a very strong opinion that they are of cosmic origin — exces- 

 sively minute meteorites. They certainly resemble very closely 

 the fine granules which frequently roughen the surface of 

 the characteristic skin of meteorites, and from their composi- 

 tion and the circumstances under which they are found there is 

 much to be said in favour of this view. I cannot, however, 

 hold it entirely proved ; there can be little doubt, from the 

 universal presence of water-logged and partially decomposed 

 pumice on the bottom, and from the constant occurrence of par- 

 ticles of volcanic minerals in the clay, that the red clay is 

 formed in a great measure by the decomposition of the lighter 

 products of submarine volcanoes drifted about by currents, and 

 finally becoming saturated with water and sinking; and it is well 

 known that both iron and nickel in a metallic state are fre- 

 quently present in minute quantities in igneous rocks. I think 

 it is conceivable that the metallic spherules may be derived from 

 this source. 



So far as we can judge, after a most careful comparative ex- 

 amination, the deposit which is at present being formed at ex- 

 treme depths in the ocean does not correspond either in structure 

 or in chemical composition with any known geological formation ; 

 and, moreover, we are inclined to believe, from a consideration 

 of their structure and of their imbedded organic remains, that 

 none of the older formations were laid down at nearly so great 

 depths— that, in fact, none of these have anything of an abyssal 

 character. These late researches tend to show that during past 

 geological changes abyssal beds have never been exposed, and 

 it seems hi^'hly probable that until comparatively recent geo- 

 logical periods such beds have not been formed. 



It appears now to be a very generally received opinion among 

 geologists — an opinion which was first brought into prominence 

 by Prof. Dana— that the "massive" eruptions which originated 

 the mountain chains which form the skeleton of our present con- 

 tinents, and the depressions occupied by our present seas date 

 from the secular cooling and contraction of the cntst of the 

 earth from a period much more remote than the deposition of 

 the earliest of the fossiliferous rocks, and that during the period 

 chronicled by the successive sedimentary systems, with many 

 minor oscillations by which limited areas have been alternately 

 elevated and depressed, the broad result has been the growth by 

 successive steps of the original mountain chains and the extension 

 of the continents by their denudation, and the corresponding deep- 

 ening of the original grooves. If this view be correct — and it cer- 

 tainly appears to me that the reasoning in its favour is very cogent 

 — it is quite possible that until comparatively recent times no 

 part of the ocean was sufficiently deep for the formation of a 

 characteristic abyssal deposit. 



Time will not allow me even to allude to the interesting 

 results which have been obtained from the determination of the 

 density of sea water from different localities and different depths, 

 and from the analysis of sea water and its contained gases, and 

 perhaps these results have been scarcely sufficiently worked out 

 as yet to afford safe bases for generalisation. I must, however, 

 say a few words as to certain additions v.hich have been made to 

 our knowledge of the two hitherto impregnable strongholds oi 

 the frost, the regions round the North and South Poles. 



The Arctic Regions. — The question which has of late held the 

 most prominent place in all discussions about the conditions of 

 the Arctic Regions, particularly since the voyage of Dr. Hayes, 

 is whether it is possible that there can be at all times or at any 

 time anything in the form of an open Polar sea. This question 

 seems now to be virtually settled, and in the most unsatisfactory 

 manner imaginable. There can be no doubt that in the year 

 1 87 1 Count Wilczek, in the schooner Isbjont, found the sea 

 between Novaya Zemlya and Spitzbergen nearly free from ice, 

 andjthat the same sea presented to Weyprecht and Payer in the 

 following year a dangerous stretch of moving and impenetrable 



