This is merely a local variety of globigerina ooze in which the comparatively large but very delicate spindle shaped shells of pteropods happen to abound. These shells do not retain their individuality at depths greater than 3,00o metres, and, in fact, pteropod ooze is only found in small patches on the ridges.
This was recognized by Sir John Murray as the characteristic deposit in high latitudes in the Indian ocean, and later it was found to be characteristic also of the correspond ing parts of the Atlantic and Pacific, covering a total area of about 23,000,000 sq. kilometres. It is in these latitudes not seldom combined with marine glacial deposits. It has been found sporadically near the Aleutian islands, between the Philippines and Marianne islands and to the south of the Galapagos group.
It is made up to a large extent of the siliceous frustules of diatoms. It is usually yellowish-grey and often straw-coloured when wet, though when dried it becomes white and mealy.
This was discovered and named by Sir Wyville Thomson on the "Challenger" in 1873 when sounding in depths of 5,000 metres and more on the way from the Canary islands to St. Thomas. The reddish colour comes from the presence of oxides of iron, and particles of manganese also occur in it, especially in the Pacific region, where the colour is more that of chocolate; but when it is mixed with globigerina ooze it is grey. Red clay is the deposit peculiar to the abysmal area; 7o carefully investigated samples collected by the "Challenger" came from an average depth of 5,000 metres. Red clay has not yet been found in depths less than 3.500 metres. The main ingredient of the deposit is a stiff clay which is plastic when fresh, but dries to a stony hardness. Isolated gritty fragments of minerals may be felt in the generally fine-grained homogeneous mass. The dredge often brings up large numbers of nodules formed upon sharks' teeth, the ear-bones of whales or turtles, or small fragments of pumice or other volcanic ejecta, and all more or less incrusted with man ganese oxide until the nodules vary in size from that of a potato to that of a man's head. A very interesting feature is the small proportion of calcium carbonate, the amount present being usually less as the depth is greater; red clay from depths exceeding 5,500 metres does not contain so much as 1% of calcareous matter. The
red clay is the characteristic deposit of the Pacific ocean, where about ioi,000,000 sq.km. are covered with it, while only 15,000,000 sq.km. of the Indian ocean and 14,000,000 sq.km. of the Atlantic are occupied by this deposit ; it is, indeed, the dominant sub marine deposit of the water-hemisphere, just as globigerina ooze is the dominant submarine deposit of the land-hemisphere.
This was recognized as a distinct deposit and named by Sir John Murray on the "Challenger" expedition, but it may be viewed as red clay with an exceptionally large pro portion of siliceous organic remains, especially those of the radio larians which form part of the pelagic plankton. It does not occur in the Atlantic ocean at all, and in the Indian ocean it is only known round Cocos and Christmas islands; but it is abundant in the Pacific.
Murray and Renard recognized a progressive diminution of carbonate of lime with increase of depth as a characteristic of all eupelagic deposits. The whole collection of 231 specimens of deep sea deposits brought back by the "Challenger" shows the following general relationship : Proportion of Calcium Carbonate in Deep Sea Deposits 68 samples from less than 2,000 fathoms (3,600 metres) =60-8o% 68 samples from 2,000-2, Soo fathoms (3,600-4,600 metres)
65 samples from 2,500-3,00o fathoms (4,600-5,500 metres) =17'4% 8 samples from more than 3,00o fathoms (5,5oo metres) =0•g% New studies of the "Meteor" expedition show that near the great depths the configuration of the floor plays an important part. Where, as a result of an elevation of the ocean floor, the water in a basin is stagnant or moves only slowly, e.g., in the Congo deep, north of the Walfisch ridge, the diminution of the calcium carbon ate with increasing depth is much less marked than where the ground currents bring a continuous inflow of new water. Thus we find in the floor deposits at about 5,000 metres depth west of the mid-Atlantic ridge in the Brazil deep only 2% of calcium car bonate, while eastward in the Congo deep we have 46%.