As I have mentioned, the soundings from the great Atlantic plain are almost entirely made up of _Globigerinae_, with the granules which have been mentioned, and some few other calcareous shells; but a small percentage of the chalky mud--perhaps at most some five per cent. of it--is of a different nature, and consists of shells and skeletons composed of silex, or pure flint. These silicious bodies belong partly to the lowly vegetable organisms which are called _Diatomaceae_, and partly to the minute, and extremely simple, animals, termed _Radiolaria_. It is quite certain that these creatures do not live at the bottom of the ocean, but at its surface--where they may be obtained in prodigious numbers by the use of a properly constructed net. Hence it follows that these silicious organisms, though they are not heavier than the lightest dust, must have fallen, in some cases, through fifteen thousand feet of water, before they reached their final resting-place on the ocean floor. And, considering how large a surface these bodies expose in proportion to their weight, it is probable that they occupy a great length of time in making their burial journey from the surface of the Atlantic to the bottom. But if the _Radiolaria_ and Diatoms are thus rained upon the bottom of the sea, from the superficial layer of its waters in which they pass their lives, it is obviously possible that the _Globigerinae_ may be similarly derived; and if they were so, it would be much more easy to understand how they obtain their supply of food than it is at present. Nevertheless, the positive and negative evidence all points the other way. The skeletons of the full-grown, deep-sea _Globigerinae_ are so remarkably solid and heavy in proportion to their surface as to seem little fitted for floating; and, as a matter of fact, they are not to be found along with the Diatoms and _Radiolaria_, in the uppermost stratum of the open ocean. It has been observed, again, that the abundance of _Globigerinae_, in proportion to other organisms of like kind, increases with the depth of the sea; and that deep-water _Globigerinae_ are larger than those which live in shallower parts of the sea; and such facts negative the supposition that these organisms have been swept by currents from the shallows into the deeps of the Atlantic. It therefore seems to be hardly doubtful that these wonderful creatures live and die at the depths in which they are found.[30] However, the important points for us are, that the living _Globigerinae_ are exclusively marine animals, the skeletons of which abound at the bottom of deep seas; and that there is not a shadow of reason for believing that the habits of the _Globigerinae_ of the chalk differed from those of the existing species. But if this be true, there is no escaping the conclusion that the chalk itself is the dried mud of an ancient deep sea.