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6°.2 Fahr. along the western coast of Scotland and its | islands, and rising to 14°5 at Fruholm near the North Cape. And it is also a very significant fact (ascertained by the careful inquiries of Mr Buchan), that while the summer isotherms cross the British Islands nearly east and west (the temperature diminishing pretty regularly from south to north), the winter isotherms traverse them nearly north and south (the temperature diminishing from west to east); whilst in Ireland the isotherms seem to envelope the islands in their folds, which increase in warmth from the centre of the island to its sea-board. So in Norway the isothermal lines run parallel to the coast-line, and this alike in summer and in winter, the temperature falling in winter, and rising in summer, with the increase of distance from the sea. Nothing could prove more conclusively than such facts as these (taken in connection with the absence of ice in the harbours of Norway, even as far north as Hammerfest, through the whole winter) the dependence of the mild winter climate of the north-western coasts of Europe upon the proximity of a sea which is warmer than the superincumbent atmosphere; and we have now to inquire how this great N.E. movement of a stratum of warm water sufficiently thick to retain a surface-temperature considerably higher than that of the air above it is to be accounted for.

The solution of the problem seems to be afforded by the doctrine of a General Oceanic Circulation, sustained by opposition of temperature only, which was first distinctly propounded in 1845 by Professor Lenz of St Petersburg, on the basis of observations made by him during the second voyage of Kotzebue (1825-1828). Others had been previously led to surmise that "Polar Currents" flow along the floors of the great oceans, even as far as the equator, balancing the superficial counter-currents which are observable in the opposite direction. But Lenz was led to conclude that the whole of the deeper portion of the great ocean-basins in communication with the polar areas is occupied by polar water, which is constantly, though slowly, flowing towards the equator; whilst conversely the whole upper stratum of equatorial water is as constantly, though slowly, flowing towards one or both of the poles. And he particularly dwelt on the existence of a belt of water under the equator, colder than that which lies either north or south of it, as an evidence that polar water is there continually rising from beneath towards the surface, a phenomenon which, he considered, admits of no other explanation. He further adduced the low salinity of equatorial water (previously noticed by Humboldt, and confirmed by his own observations), compared with that of tropical water, as evidence that the equatorial water of the surface is derived from the polar underflow. And he attributed the maintenance of this circulation to the continually renewed disturbance of equilibrium between the polar and equatorial columns, the greater lateral (because downward) pressure of the former1 causing a bottom outflow of polar water in the direction of the latter, whilst the reduction of level thus occasioned will produce a surface indraught from the warmer towards the colder areas.

The doctrine of Lenz, so far from meeting with the general acceptance to which it had a fair claim,-alike on theoretical grounds and from its accordance with the facts ascertained by careful observation,-seems to have been put aside and forgotten, a preference being given to the doctrine of the prevalence of a uniform deep-sea temperature of 39°, which was supposed to be established by the

1 It must be borne in mind that sea water does not expand like fresh water in cooling below 39° 2, but continues to contract down to its freezing point, which lies between 27° and 25° Fahr., according as it is still or agitated.

thermometric observations made in the voyages of D'Urville and Sir James Ross. No such precaution was taken, however, in these observations as that to which Lenz had recourse, to obviate the effects of the tremendous pressure (1 ton per square inch for every 800 fathoms of depth) to which deep-sea thermometers are exposed; and it is now certain that the temperatures at great depths recorded by D'Urville and Ross were several degrees too high.

This

It was in entire ignorance of the doctrine of Lenz, and under the influence of that of D'Urville and Ross, which had been stamped with the great weight of Sir John Herschel's weight of authority,2 that Dr Carpenter commenced in 1868 (in concert with Professor Wyville Thomson) a course of inquiry into the thermal condition of the deep sea, which at once convinced him of the fallacy of the uniform 39° doctrine, and led him to conclusions essentially accordant with those of Lenz. For in the channel of from 500 to 600 fathoms' depth between the north of Scotland and the Faroe Islands, they found the deeper half to be occupied by a stratum of glacial water, whose temperature ranged downwards from 32° to 29°5 ; whilst the upper half was occupied by a stratum warmer than the normal temperature of the latitudes. phenomenon was interpreted by Carpenter as indicating a deep glacial flow from N.E. to S.W., and a warm upper flow from S. W. to N.E.; and finding that to the west of this channel, on the border of the deep Atlantic basin, the excess of warmth extended to a depth of more than 500 fathoms, he came to the conclusion that the north-moving stratum which brought it could not be an extension of the true Gulf Stream, but must be urged on by some much more general force. A series of temperature-soundings taken along the west of Ireland, the Bay of Biscay, and the coast of Portugal, confirmed him in this view, by showing that the division between an upper warm stratum and a cold under-stratum exists in the North Atlantic at a depth of from 700 to 900 fathoms, the whole mass of water below this having either flowed into the basin from the polar area, or having had its temperature brought down to from 39° to 36°5 by mixture with the polar inflow. And this conclusion was confirmed by the result of temperature-soundings taken at corresponding depths and under the same parallels of latitude in the Mediterranean; for as they showed a uniform temperature of from 54° to 56°, from beneath the stratum of 100 fathoms that was superheated by direct insolation, to the very bottom, it became clear that depth per se could have no effect in reducing the bottom-temperature; and that the cause of the excess of temperature in the mass of water occupying the Mediterranean basin above that of Atlantic water at the same depths, lies in the seclusion of the former from the polar underflow which brings down the deep temperature of the latter. This conclusion having received marked confirmation from temperature-soundings taken in the Eastern seas, was put forward by Carpenter as justifying the doctrine of a vertical oceanic circulation sustained by opposition of temperature only, quite independent of and distinct from the horizontal circulation produced by wind,-which doctrine he expressed in terms closely corresponding with those that had been used by Lenz. And the collection of data for the establishment or confutation of this doctrine was one of the objects of the "Challenger" expedition, which has already made, in the determination of the thermal stratification of the Atlantic between 38° N. lat. and 38° S. lat., what may be fairly characterised as the grandest single contribution ever yet made to terrestrial physics.

The following are the most important of the facts thus 2 See his Physical Geography of the Globe, originally published in the eighth edition of this Encyclopædia.

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established: Of the water which fills the deep trough of | and St Thomas (lat. 181° N.), divided by the "Dolphin the North Atlantic (fig. 1) between Teneriffe (lat. 281 N.) rise" into an eastern and western basin, by far the larger

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Horizontal Scale of Nautical Miles
FIG. 1.-Section of North Atlantic Ocean between St Thomas and Teneriffe.
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mass has a temperature ranging from 40° downwards, in
the eastern basin, to a bottom-temperature of 3510, whilst
in the western basin-apparently under the influence of
the Antarctic underflow-the bottom-temperature sinks to
34° 4. A tolerably regular descent is shown in this sec-
tion, from a surface-temperature rising near St Thomas to
75°, to the bathymetrical isotherm of 45°, which lies be-
tween 400 and 600 fathoms' depth; there is then a stratum
between 45° and 40°, of which the thickness varies from
about 250 to 450 fathoms, the isotherm of 40° lying at
between 750 and 1000 fathoms' depth, while below this,
down to the bottom at between 2000 and 3000 fathoms,
the further reduction to 34°4 is very gradual.

The same general condition prevails in the South Atlantic (fig. 2), between Abrolhos Island (lat. 18° S.) on the coast of Brazil, and the Cape of Good Hope (lat.

34° S.), this trough also being divided into two basins by the elevation of the bottom which culminates in the island of Tristan da Cunha. The temperature of the water that occupies it, however, is lower through its whole vertical range than that of the North Atlantic. The stratification is nearly uniform from the surface downwards to the isotherm of 40°, which lies at from 300 to 450 fathoms' depth, the isotherms of 39° and 38° also lying within about 500 fathoms; there is then a slower reduction down to the isotherm of 35°, which lies between 1400 and 1800 fathoms; while the whole sea-bed is covered by a stratum of about 600 fathoms' thickness, whose temperature ranges downwards from 35° to 33°. The whole of this deepest stratum is colder than any water that is found in the corresponding portion of the North Atlantic, except near St Thomas

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Horizontal Scale of Nautical Miles.

FIG. 2.-Section of South Atlantic.

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It is not a little remarkable that the sub-surface stratum | North Atlantic (fig. 1), and 100 fathoms nearer to the of water, having a temperature above 40°, is thinner under the equator than it is in any other part of the Atlantic from the Faroe Islands to the Cape of Good Hope. Notwithstanding the rise of the surface-temperature to 76°-80°, the thermometer descends in the first 300 fathoms more rapidly than anywhere else; so that polar water is met with, as shown in fig. 3, at a much less depth than in the

surface than even in the colder South Atlantic (fig. 2); whilst the temperature of the bottom is but little above 32°. Thus the influence of the polar underflow is more pronounced under the equator than it is elsewhere; as is distinctly seen in the section shown in fig. 4, which is taken in a north and south direction so as to exhibit the relation of the thermal stratification of the North to that

of the South Atlantic, and of both to that of the equatorial depth of about 700 fathoms, gradually rises as the equator

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is approached; and it is between the equator and 7° S., where the surface-temperature rises to nearly 80°, that cold water is soonest reached, the isotherm of 40° rising to within 300 fathoms of the surface, while that of 55°, which in lat. 38° N. lies at nearly 400 fathoms' depth, and in lat. 22° N. at about 250 fathoms, actually comes up under the equator within 100 fathoms of the surface. At the same time, while the bottom-temperature under the equator is the lowest anywhere met with, namely, 32° 4,1 the thickness of the stratum beneath the isotherm of 35° is not less than 600 fathoms. In passing southwards, the superficial isotherms are observed to separate again from each other, partly by the reduction of the surface-temperature, and partly by the descent of the isotherm of 40° to a depth of something less than 400 fathoms, which it keeps with little reduction as far south as the Cape of Good Hope. The significance of these facts becomes more remarkable, when we consider that if a portion of the oceanic area under the

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belt. The isotherm of 40°, which in lat. 22° N. lies at a equator were to be secluded, like the Mediterranean or the

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Haizontal Scale af Nautical Miles
FIG. 4.-Section of Mid-Atlantic, taken nearly north and south.

Red Sea, from all but local influences, the temperature of
its water from the sub-surface stratum downwards to the
bottom-whatever its depth-would be its isocheimal or
mean winter-temperature, which, in the equatorial zone,
would be certainly not below 75°.

Nothing, Dr Carpenter contends, could more conclusively support the general doctrine of a Vertical Oceanic Circulation sustained by opposition of temperature, than the precise conformity of the facts thus determined by observation to the predictions which his confidence in the theory had led him to put forth. These predictions were essentially as follows:

"1. That instead of the local depressions of bottom-temperature imputed by previous writers to polar currents, the temperature of every part of the deep sea-bed in communication with either of the polar areas would be not many degrees above that of the polar areas themselves. "2. That this general depression of bottom-temperature would be found to depend, not upon such a shallow glacial stream as might be maintained to be a return from the polar areas of water propelled towards them by wind-currents, but upon a creeping flow of the whole under-stratum, having a thickness of from 1000 to 2000

fathoms.

"3. That as the depression of bottom-temperature in any part of the general oceanic basin would be proportional to the freedom of communication between its deeper portion and that of one or other of the polar areas, the bottom-temperature of the South Atlantic would probably range downwards to 32°, while that of the North Atlantic would not be below 35°, except where it first receives the Arctic flow, or comes under the influence of the Antarctic underflow, which would very probably extend itself to the north of the equator. "4. That as the Arctic and Antarctic underflows must meet at or near the equator, whilst the surface-stratum is there continually being draughted off thence towards either pole, there would be a

continual ascent of glacial water under the line, showing itself by a nearer approach of cold water to the surface in the inter-tropical than in the extra-tropical zone.'

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It was further pointed out by Lenz, and more recently (in ignorance of his doctrine) by Carpenter, that additional evidence of such ascent is furnished by the low salinity of the surface-water of the equatorial belt corresponding with that of polar water. For, as was originally observed by Humboldt, then by Lenz himself, and subsequently by many other voyagers, the specific gravity of the surfacewater of the Atlantic gradually increases as either tropic is approached from the polar side of its own hemisphere, reaches its maximum a little nearer the equator, and then rapidly diminishes, coming down under the equator to the standard of polar water. Thus a mean of eight observations taken in the "Challenger" expedition between Bermuda (32° N.) and St Thomas (18° N.) gave 1027-2 as the sp. gr. of surface-water, whilst a mean of seventeen observations between the Cape Verd Islands (16° N.) and Bahia (13° S.) gave a sp. gr. of only 1026 3. Now, since between St Thomas and Bermuda the eight "Challenger" observations of bottom (polar) water gave a mean sp. gr. of 1026-3, whilst between Cape Verd and Bahia the mean sp. gr. of the bottom-water was even slightly lower (the results being

1 That the bottom-temperature beneath the equator was lower than any that was met with in the South Atlantic, is attributable to the circumstance that, in consequence of unfavourable weather, the temperature-soundings were taken at intervals too wide to detect the deep channel through which the coldest Antarctic water doubtless flowed towards the equator.

in all cases expressed according to a common standard of temperature), such a close conformity subsists between the salinity of the equatorial water of the surface and that of the polar waters of the bottom, as can scarcely be accounted for in any other way than by the continual and tolerably rapid ascent of the latter.

Another indication of this ascent is given by the moder ation of the surface-temperature of oceanic water, even under the equator. If there were no ascent of colder water from beneath, there seems no reason why the constant powerful insolation to which equatorial water is subjected should not raise the temperature of its surface to the highest possible elevation. The limit to that elevation, which is obviously set by the cooling influence of evaporation, is probably that which is met with in the Red Sea, where the monthly average for August rises to 86° and for September to 88°, whilst the maxima rise much higher, temperatures of 100°, 106°, 100°, and 96° having been noted on four consecutive days. Moreover, along the Guinea Coast, and especially in the Bight of Biafra, the surfacetemperature is stated to range as high as 90°. But in these cases there is no reduction of surface-temperature by the upward movement of polar water; for this is altogether excluded from the Red Sea by the shallowness of the Strait of Babelmandeb, whilst the depth of the bottom along the Guinea Coast is too small to allow of its being overflowed by the glacial stratum. Now, over the deeper parts of the equatorial Atlantic the surface-temperature usually ranges between 75° and 80°; and this is its ordinary range in the Mediterranean during the months of August and September. That the temperature of an equatorial ocean should be thus kept down to that of a sea of which the greater part lies between the parallels of 40° and 35°, can scarcely be accounted for in any other way than by the continual uprising of polar waters from beneath.

The same principle, once admitted, fully accounts for that amelioration of the cold of north-western Europe, which (as already shown) cannot be fairly attributed to the Florida Current or true Gulf Stream. For it is obvious that a continual efflux of the lower stratum from the polar areas towards the equatorial must involve a continual indraught of the upper stratum towards the polar areas; and this indraught will be much more marked in the Northern than in the Southern Atlantic, on account of the progressive narrowing of the former, whilst the latter progressively widens out. Of such a slow northerly set of a stratum of water, extending downwards to a depth of at least 600 fathoms, we have evidence in a comparison of the temperature-soundings taken in the "Porcupine" expedi

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seen that, although the surface-temperature is reduced by the thinning-out of the superficial stratum, there is but a slight change in the position of the bathymetrical isotherms of 45° and 40°; so that there is an obvious continuity of a stratum of many hundred fathoms' thickness between these two points, notwithstanding their separation by 25° of latitude. The contrast between the position of the isotherm of 40° at 800 fathoms' depth off the Faroes, and its position at less than 300 fathoms' depth under the equator, is most remarkable. We have seen that the isocheimal in the latter area would not be below 75°, and yet we find water colder than 40° lying at within 300 fathoms of the surface; whilst, on the other hand, the normal isocheimal at 59 N. would certainly be below 40° (probably no more than 35°), and yet we find water above 43° extending downwards to 600 fathoms, and water above 40° to 800 fathoms. Thus the vertical oceanic circulation carries a vast mass of water which is below the normal off the coast of Portugal, into a region where it is above the normal, with very little loss of heat by the way, except in its surface-film; and a little consideration will show that such a movement must be much more effectual as a heater than a corresponding movement of a thin stratum of much warmer water. For the latter, when it passes beneath an atmosphere much colder than itself, will soon be brought down to a like standard, not having warmer water from below to take its place when it has been cooled down; whilst in the former, each surface-layer, when cooled below the temperature of the warmer stratum beneath, will sink and be replaced by it. Now since the true Gulf Stream, when we last know it, has been so thinned out that it could not long retain any excess of temperature, it seems inconceivable that it should exert any decided effect on the temperature of the Faroes and the coast of Norway, unless (as supposed by Dr Petermann and Professor Wyville Thomson) its thickness undergoes an increase from less than 100 fathoms to 600. But since the course of Dr Petermann's isotherms shows that the northward flow extends across the whole breadth of the Atlantic between Newfoundland and the British Isles-a distance of about 2000 miles-we are required to believe that a rivulet (for such it is by comparison) of 60 miles' breadth and 100 fathoms' depth (see section, fig. 5), of which the greater part turns southwards round the Azores, and of which the remainder is flowing due east when we last recognise it, is able to impart a northerly movement to a stratum of 2000 miles in breadth, and at least 600 fathoms' depth. On the other hand, the eastward set of this stratum, considered as a northward indraught into the polar area, is readily accounted for by the excess of easterly momentum which it derives from the earth's rotation, this being only half as rapid in lat. 60° as it is under the equator; and since there is a still more rapid reduction in the rate of this rotation in yet higher latitudes, the continually increasing excess of easterly momentum will give to the northward flow a progressively stronger eastward set.

On the other hand, the deficiency of easterly momentum in the cold underflow coming from the pole towards the equator will tend to produce a lagging-behind, or westward set of that underflow; and this has been shown by the "Challenger" temperature-soundings to be the case,-the cold deep strata of the Western Atlantic surging upwards along the slope of the North American coast-line, as is shown in fig. 6, where we see not only the bathymetrical isotherms of 60°, 55°, and 50°, but the yet deeper isotherms of 45° and 40°, successively rising to the surface as we approach the land; while at a depth of only 83 fathoms, a temperature of 35° was encountered, which, at no great distance to the south, would only be found at a depth of 2000 fathoms. That the cold water should thus run up.

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hill is quite conformable to what we see in other cases, in | than that of anchorage), the enormous mass of Gulf-weed which a heavier under-stratum has a definite set towards a

slope; and whilst the existence of such a westerly set is, ex hypothesi, a necessary consequence of the southerly movement of the Arctic underflow, no other explanation of

it has been suggested. We now see that the cold Labrador Current overlies a band of water as cold as itself; and the south

ward extension of this cold band, far beyond that of any definite

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Vertical Scale of Fathoms.

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FIG. 6. Section from Bermuda to Halifax.

640 Bermuda,

ment, and its entrance into the Gulf of Mexico, through the Florida Channel, at the side of and beneath the outflowing Gulf Stream, are thus accounted for.

The remarkable accordance of so many facts of actual observation, in the Atlantic area, with the probabilities deducible from a theory whose soundness can scarcely be disputed, seems now to justify the admission of the general (vertical) oceanic circulation sustained by opposition of temperature as an accepted doctrine of terrestrial physics. Distribution of Organic Life.-All that will be attempted under this head will be to indicate the general conditions that seem, from recent researches, to have the greatest influence on the distribution of plants and animals through this great oceanic basin.

The distribution of marine plants seems mainly determined by light, temperature, and depth,—a further influence being exerted by the character of the shores. The diminution of light in its passage through sea-water is so rapid, that the quantity which penetrates to a depth of 250 or 300 fathoms may be regarded as almost infinitesimal; and in conformity with this we find a very rapid diminution of Algal life below the depth of 100 fathoms. The upper stratum is occupied for the most part by the larger and coarser forms of the Fucaceae, or olive-green sea-weeds, whilst the more delicate Ceramiacea, or red sea-weeds, frequent deeper waters; and, as it appears from experiments made in aquaria that the latter do not flourish in full light, but grow well in shadow, it may be concluded that their preference for a moderate depth is rather for reduced light and stillness than for depth per se. At a depth of 150 fathoms very few ordinary sea-weeds maintain their ground; and below this we seldom find any Algae, save the Corallines and Nullipores consolidated by calcareous deposit. The distribution of particular types over different parts of the Atlantic area appears to be mainly regulated by temperature; and this would seem to be remarkably the case with the floating Diatomacea, which, though they form green bands in the surface-water of polar seas, have not been encountered in like abundance in the Atlantic, and do not contribute largely, by the subsidence of their siliceous lorica, to the composition of its bottom-deposit. Although it is the habit of the larger Algae to grow from a base of attachment (their roots serving no other purpose however,

found in the Sargasso Sea seems quite independent of any such attachment. It was at one time supposed that this originally grew on the Bahama and Florida shores, and was torn thence by the powerful current of the Gulf Stream; but it seems certain that if such was its original source, the "Gulf-weed" now lives and propagates whilst freely floating on the ocean-surface, having become adapted by various modifications to its present mode of existence.

The distribution of the animals that habitually live in that upper stratum of the ocean whose degree of warmth varies with the latitude, seems mainly determined by temperature. Thus the "right whale" of Arctic seas, and its representative in the Antarctic, seems never to enter the inter-tropical area, generally keeping away from even the temperate seas, whilst, on the other hand, the sperm-whale ranges through the parts of the ocean where the "right whales are never seen. The distribution of fishes seems generally to follow the same rule; as does also that of floating mollusks. Thus the little Clio (a Pteropod mollusk), which is a principal article of the food of the "right whales" in polar seas, is rarely met with in the Atlantic, where, however, other pteropods, as Hyalaa, present themselves in abundance. On the other hand, the warmer parts of its area swarm with Salpa-chains, which are not frequent in higher latitudes; and the few representatives of the Nautiloid Cephalopods, that were so abundant in Cretaceous seas, are now restricted to tropical or sub-tropical areas. And the distribution of the mollusks, echinoderms, and corals, which habitually live on the bottom, seems to be determined, within certain limits at least, by temperature rather than by depth.

And

The bathymetrical range to which animal life of any higher type than the Rhizopodal might extend, was until recently quite unknown; but the researches initiated by Prof. Wyville Thomson and Dr Carpenter in 1868, and since prosecuted by the "Challenger" expedition, have fully established the existence of a varied and abundant fauna in ocean-depths ranging downwards to 2000 fathoms. these researches have further established that the distribution of this fauna is mainly determined by the temperature of the sea-bed; so that whilst in the channel between the north of Scotland and the Faroes there were found at the same depths, and within a few miles of each other, two faunæ almost entirely distinct-one a boreal and the other a warmer-temperate on sea-beds having respectively the temperatures of 30° and 43°, various types to which a low temperature is congenial are traceable continuously along the whole abyssal sea-bed that intervenes between those northern and southern polar areas within which they present themselves at or near the surface. And hence it becomes clear that, since glacial types are even now being embedded in the strata which are in process of formation beneath the equator, no inferences as to terrestrial climate can be drawn from the character of marine deposits.

One very remarkable feature which presents itself over a large proportion of the Atlantic basin is the abundance of the minute Globigerina and other Foraminifera, the accumulation of whose shells, and of their disintegrated remains, is giving rise to a calcareous deposit of unknown thickness, that corresponds in all essential particulars to Chalk. This deposit, in some parts of the North Atlantic, is replaced by an Arctic drift of fine sand, whilst in other parts there is a mixture of arenaceous and of calcareous components, such as is found in certain beds of the Cretaceous formation. Now on the surface of this deposit there have been found so many living types, especially belonging to the groups of Echinoderms, Corals, Siliceous Sponges, and Foraminifera, which closely correspond with types hitherto regarded as characteristic of the Cretaceous epoch, that the question naturally suggests itself whether

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