Page:Popular Science Monthly Volume 44.djvu/484

470 depths ranging from 500 to 2,800 fathoms of water in equatorial and temperate latitudes. The reason that it is not found in arctic seas may be that the cold surface waters of these regions do not bear such an abundant fauna of foraminifera. This is supported by the fact that it extends ten degrees farther north than south in the Atlantic, the warm water of the Gulf Stream bearing a richer fauna than the waters of a corresponding degree of latitude in the southern sea.

The pteropod ooze has only twenty-five per cent of carbonate of lime. It contains numerous shells of various pteropods, heteropods, and foraminifera, but nearly fifty per cent of its substance is composed of the siliceous skeletons of radiolaria and the frustules of diatoms. According to Murray, it is found in tropical and subtropical seas at depths of less than 1,500 fathoms.

The radiolarian ooze is found only in the deepest waters of the central and western Pacific Ocean. In some of the typical examples not a trace of carbonate of lime was to be found, but in somewhat shallower waters a few small fragments occurred. A diatom ooze, mainly composed of the skeletons of diatoms, has also been found in deep water near the Antarctic Circle, but it has not apparently a very wide range.

Of all the deep-sea deposits, however, the so-called "red mud" has by far the widest distribution. It is supposed to extend over one third of the earth's surface. It is essentially a deep-sea deposit, and one that is found in its typical condition at some considerable distance from continental land. Like the globigerina ooze it is never found in inclosed seas. To the touch it is plastic and greasy when fresh, but it soon hardens into solid masses. When examined with the microscope it is seen to be composed of extremely minute fragments, rarely exceeding 0·05 millimetre in diameter. It contains a large amount of free silica that is probably formed by the destruction of numerous siliceous skeletons, and a small proportion of silicate of alumina. It usually contains the remains of diatoms, radiolaria, and sponge spicules, and occasionally lumps of pumice stone, meteoric nodules, and, in colder regions, stones and other materials dropped by passing icebergs.

In the great oceans, then, we find in the deepest places red mud, or, where there is an abundant radiolarian surface fauna, radiolarian ooze; in water that is not deeper than about 2,000 fathoms, we find the globigerina ooze; in shallower waters and in some localities only pteropod ooze. It must not be supposed that sharp limits can anywhere be drawn between these different kinds of deposits, for they pass gradually into one another and present many intermediate forms.

It is probable that the sea water, by virtue of the free carbonic acid it contains in solution, is able to exert a solvent action upon