Page:Popular Science Monthly Volume 7.djvu/47

Rh. Nor can the presence of the soft parts of the body in the shells which form the Globigerina ooze, and the fact, if it be one, that animals living at the bottom use them as food, be considered as conclusive evidence that the Globigerinæ live at the bottom. Such as die at the surface, and even many of those which are swallowed up by other animals, may retain much of their protoplasmic matter when they reach the depths at which the temperature sinks to 34° or 32° Fahr., where decomposition must become exceedingly slow.

Another consideration appears to me to be in favor of the view that the Globigerinæ and their allies are essentially surface-animals. This is the fact brought out by the Challenger's work, that they have a southern limit of distribution, which can hardly depend upon any thing but the temperature of the surface-water. And it is to be remarked that this southern limit occurs at a lower latitude in the antarctic seas than it does in the North Atlantic. According to Dr. Wallich ("The North-Atlantic Sea-Bed," p. 157) Globigerina is the prevailing form in the deposits between the Farœ Islands and Iceland, and between Iceland and East Greenland—or, in other words, in a region of the sea-bottom which lies altogether north of the parallel of 60° north; while in the southern seas the Globigerinæ become dwarfed and almost disappear between 50° and 55° south. On the other hand, in the sea of Kamtchatka, the Globigerinæ have vanished in 56° north, so that the persistence of the Globigerina ooze in high latitudes, in the North Atlantic, would seem to depend on the northward curve of the isothermals peculiar to this region; and it is difficult to understand how the formation of Globigerina ooze can be affected by this climatal peculiarity unless it be effected by surface animals.

Whatever may be the mode of life of the Foraminifera, to which the calcareous element of the deep-sea "chalk" owes its existence, the fact that it is the chief and most widely-spread material of the sea-bottom in the intermediate zone, throughout both the Atlantic and Pacific Oceans, and the Indian Ocean, at depths from a few hundred to over 2,000 fathoms, is established. But it is not the only extensive deposit which is now taking place. In 1853 Count Pourtalès, an officer of the United States Coast Survey, which has done so much for scientific hydrography, observed that the mud forming the sea-bottom at depths of 150 fathoms, in 31° 32' north, and 79° 35' west, off the coast of Florida, was "a mixture, in about equal proportions, of Globigerinæ and black sand, probably green sand, as it makes a green mark when crushed on paper." Prof. Bailey, examining these grains microscopically, found that they were casts of the interior cavities of Foraminifera, consisting of a mineral known as Glauconite which is a silicate of iron and alumina. In these casts the minutest cavities and finest tubes in the Foraminifera were sometimes reproduced in solid counterparts of the glassy mineral, while the calcareous original had been entirely dissolved away.