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The observed increase of density with increasing depth is chiefly attributable, down to iooo fathoms, to decrease of temperature, but below that depth the continued rise in density is due to a small increase of salinity which prevails in the greater depths of the ocean, augmented by the compression to which the deep waters are subject under the weight of the superincumbent mass, since water is compressible by about ^Vo" of its bulk under the pressure of one atmosphere. Leaving out of account the pressure of the atmosphere upon the surface, the pressure at successive depths in the ocean is as follows:

Pressure per square inch
Depth Atmosphere Pounds Tons

33 feet I 15

66" 2 30

99" 3 45

100 fathoms 18 270

500" 90 I3SO

1000" 180 2700 1.2

2000" 360 5400 2.4

3000" 540 8100 3.6'

4000" 720 10800 48

5000 *' 900 13500 60

S348" 960 14400 6.4

The viscosity or internal friction of sea water, although varying but little with change of salinity within the common limits, varies greatly with change of temperature. If the viscosity of fresh water at 320 F. be stated as 100, the viscosity of sea water having a salinity of 30 parts per thousand will be 102 at 320 F. and 52 at 770 F., and the viscosity of sea water having a salinity of 35 parts per thousand will be 103 at 32° F. and 53 at 770 F. This indicates that the same body would sink twice as fast in sea water at a temperature of 770 F. as it would in the same water at 320 F.

The blue color of the ocean is owing to the selective absorption of light by sea water, in which the blue rays are ten times less absorbed than the red rays. There is light entering into the lesser depths of the ocean, but it is different from the familiar light of day on account of its deficiency in the rays from the red end of the solar spectrum. Recent observations conducted with photographic plates showed that, on a sunny day in the Sargasso Sea, there was sufficient light at a depth of 550 fathoms to affect a sensitive film after an exposure of 80 minutes. Another plate was exposed for 2 hours at a depth of 900 fathoms without showing any effect. By employing filters for rays of different color, it was shown that, at a depth of 275 fathoms, many blue rays were present but scarcely any red ones, while at a depth of 55 fathoms all the component rays of sunlight were present, although there were fewer of the red than the others. The extent of the vegetable kingdom in the ocean is confined to this region of lightpenetration or photic zone which must vary in its depth according to the zenith distance of the luminary and the nature of the materials held in suspension and solution in the water. The living matter which covers the globe wherever water, air, and earth commingle, while extending only a short depth into the crust of the earth and a short height into the atmosphere, extends throughout the whole depth of the ocean; but below the photic zone all of the numerous forms of living things which abound belong to the animal kingdom.

The deposits which cover the bed of the ocean have been classified by Murray, according to their origin, into two grand divisions —terrigenous and pelagic. In the accompanying table these deposits are described and further subdivided into classes, and the distribution of the principal ones are shown in the subjoined map: f Boulders, shingle.^

Littoral Deposits between high and low water marks.

Shallow-water Deposits, between low water mark and 100 fathoms.

DeepSea Deposits beyond 100 fathoms.

gravels, sands,
muds, etc., de-
rived from ad-
jacent land.

Sands, gravels,
muds, marls, de-
rived from adja-
cent land, shores,
and shallow
waters.

Blue mud,
Red mud,
Green mud,
Volcanic mud,
Coral mud,

Globigerina ooze,
Pteroped ooze,
Diatom ooze,
Radiolarian ooze,
Red clay.

Terrigenous Deposits formed in deep and shallow water close to land masses, from materials carried down from the land surfaces or torn away from the coast together with the remains of organisms which live on the bottom in shallow waters.

Pelagic Deposits formed in deep water far removed from land, largely from the remains of calcareous and siliceous organisms which have lived in the surface waters and have fallen to the bottom after death.

As yet knowledge is scanty in regard to the rate of deposition and the thickness of marine deposits, since, in the operation of deep-sea soundings, the bottom soil has not usually been penetrated more than a few inches. It is very doubtful wrhether any analogues to the terrestrial rocks are to be found in connection with the pelagic deposits, from which there is a significant absence of quartz. They are composed largely of the remains of organisms which have lived in the surface and sub-surface waters of the ocean and whose remains after death have fallen gradually through the depths of the ocean to the bottom. During the course of this descent, the shell of a lime-secreting organism is exposed

[graphic]

Deep Sea Deposits.

to the solvent action of sea water; and more and more of it is dissolved away as greater and greater depths are reached, so that deep-sea deposits are, in a measure, characterized by the amount of carbonate of lime that they contain, as illustrated in the accompanying diagram.

It has been pointed out that there exist in the ocean systematically varying distributions of salinity and of temperature, giving rise to a systematic distribution of the density or heaviness of the waters. This is one of the basic factors in oceanic circulation, causing a systematic descent from the surface of the heavier waters which characterize the polar borders of the temperate zones, and a slow movement thence of the deeper waters, entirely distinct from the oscillations of the mass of the ocean Set up by seiches and tides, both toward the equator and toward the poles, where ascending currents rise from the depths. The characteristics of this system of circulation, which is illustrated in the accompanying diagram, are confirmed by the preceding temperature profiles of the Atlantic and Pacific oceans in which the serpentine arrows indicate the regions of ascending circulation and the fair arrows the regions of descending circulation, and yet further by the section showing the distribution of oxygen in the depths of the Atlantic, through which it is evident that waters in the depths of

[graphic]

Diagram Showing Gradual Disappearance Of.Calcium Carbonate With Increasing Depth.

the ocean in the equatorial regions have been much longer out of contact with the atmosphere than the waters of the temperate latitudes which, descending from recent contact with the atmosphere, contain by absorption about three times as much oxygen as the deep equatorial waters which have ascended from the depths at the close of the long circuit of the bottom waters toward the equator.

[graphic]

Diagram Showing The General Circulation Of The Waters Of Thf. Atlantic Ocean (continuous Arrows Indicate Relatively Warm Water And Dotted Arrows Relatively Cold Water).

These massive movements are in striking contrast with the relatively rapid rate of motion of the surface currents impelled, for the main part, in a clockwise circuit in each of the oceans of the northern hemisphere and in an anticlockwise circuit in each of the oceans of the southern hemisphere, in general accord—as may be seen from the accompanying illustrative chart—with the circuit of the winds, which, in their direct and indirect effects taken in conjunction with the configuration of the continents, constitute the prime agency in originating and maintaining the circulation of the ocean.

As yet knowledge is scanty in regard to the rate of deposition and the thickness of marine deposits, since, in the operation of deep-sea soundings, the bottom soil has not usually been penetrated more than a few inches. It is very doubtful whether any analogues to the terrestrial rocks are to be found in connection with the pelagic deposits, from which there is a significant absence of quartz. They are composed largely of the remains of organisms which have lived in the surface and sub-surface waters of the ocean and whose remains after death have fallen gradually through the depths of the ocean to the bottom. During the course of this descent, the shell of a lime-secreting organism is exposed

[graphic][merged small]

to the solvent action of sea water; and more and more of it is dissolved away as greater and greater depths are reached, so that deep-sea deposits are, in a measure, characterized by the amount of carbonate of lime that they contain, as illustrated in the accompanying diagram.

It has been pointed out that there exist in the ocean systematically varying distributions of salinity and of temperature, giving rise to a systematic distribution of the density or heaviness of the waters. This is one of the basic factors in oceanic circulation, causing a systematic descent from the surface of the heavier waters which characterize the polar borders of the temperate zones, and a slow movement thence of the deeper waters, entirely distinct from the oscillations of the mass of the ocean Set up by seiches and tides, both toward the equator and toward the poles, where ascend

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