But the slice of chalk presents a totally different appearance when placed under the microscope. The general mass of it is made up of very minute granules; but imbedded in this matrix are innumerable bodies, some smaller and some larger, but on a rough average not more than a hundredth of an inch in diameter, having a well-defined shape and structure. A cubic inch of some specimens of chalk may contain hundreds of thousands of these bodies, compacted together with incalculable millions of granules.
The examination of a transparent slice gives a good notion of the manner in which the components of the chalk are arranged, and of their relative proportion. But by rubbing up some chalk with a brush in water and then pouring off the milky fluid, so as to obtain sediments of different degrees of fineness, the granules and the minute rounded bodies may be pretty well separated from one another, and submitted to microscopic examination, either as opaque or as transparent objects. By combining the views, obtained in these various methods, each of the rounded bodies may be proved to be a beautifully constructed calcareous fabric, made up of a number of chambers communicating freely with one another. The chambered bodies are of various forms. One of the commonest is something like a badly grown raspberry, being formed of a number of nearly globular chambers of different sizes congregated together. It is called Globigerina, and some specimens of chalk consist of little else than Globigerinæ and granules. Let us fix our attention upon the Globigerina. It is the spore of the game we are tracking. If we can learn what it is and what are the conditions of its existence, we shall see our way to the origin and past history of the chalk.
The history of the discovery of these living Globigerinæ, and of the part which they play in rock-building, is singular enough. It is a discovery which, like others of no less scientific importance, has arisen incidentally out of work devoted to very different and exceedingly practical interests. When men first took to the sea, they speedily learned to look out for the shoals and rocks; and the more the burden of their ships increased, the more imperatively necessary it became for sailors to ascertain with precision the depth of the waters they traversed. Out of this necessity grew the use of the lead and sounding-line; and ultimately marine surveying, which is the recording of the form of coasts and of the depth of the sea, as ascertained by the sounding-lead, upon charts.
At the same time it became desirable to ascertain and to indicate the nature of the sea bottom, since this circumstance greatly affects its goodness as holding-ground for anchors. Some ingenious tar, whose name deserves a better fate than the oblivion into which it has fallen, attained the object by "arming" the bottom of the lead with a lump of grease, to which more or less of the sand or mud or broken shells, as the case might be, adhered, and was brought to the surface. But however well adapted such an apparatus might be for rough nautical purposes, scientific accuracy could not be expected from the armed lead; and to remedy its defects (especially when applied to sounding in great depths), Lieutenant Brooke of the American Navy some years ago invented a most ingenious machine, by which a considerable portion of the superficial layer of the sea bottom can be scooped out and brought up from any depth to which the lead descends. In 1853 Lieutenant Brooke obtained mud from the bottom of the North Atlantic, between Newfoundland and the Azores, at a depth of more than 10,000 feet or two miles, by the help of this sounding apparatus. The specimens were sent for examination to Ehrenberg of Berlin and to Bailey of West Point; and those able microscopists found that this deep-sea mud was almost entirely composed of the skeletons of living organisms—the greater proportion of these being just like the Globigerinæ already known to occur in the chalk.
Thus far the work had been carried on simply in the interests of science; but Lieutenant Brooke's method of sounding acquired a high commercial value when the enterprise of laying down the telegraph cable between this country and the United States was undertaken. For it became a matter of immense importance to know not only the depth of the sea over the whole line along which the cable was to be laid, but the exact nature of the bottom, so as to guard against chances of cutting or fraying the strands of that costly rope. The Admiralty consequently ordered Captain Dayman, an old friend and shipmate of mine, to ascertain the depth over the whole line of the cable and to bring back specimens of the bottom. In former days, such a command as this might have sounded very much like one of the impossible things which the young prince in the fairy tales is ordered to do before he can obtain the hand of the princess. However, in the months of June and July, 1857, my friend performed the task assigned to him with great expedition and precision, without, so far as I know, having met with any reward of that kind. The specimens of Atlantic mud which he procured were sent to me to be examined and reported upon.
The results of all these operations is, that we know the contours and the nature of the surface soil covered by the North Atlantic for a distance of I,700 miles from east to west, as well as we know that of any part of the dry land. It is a prodigious plain—one of the widest and most even plains in the world. If the sea were drained off, you might drive a wagon all the way from Valentia on the west coast of Ireland, to Trinity Bay in Newfoundland; and except upon one sharp incline about 200 miles from Valentia, I am not quite sure that it would even be necessary to put the skid on, so gentle are the ascents and descents upon that long route. From Valentia the road would lie downhill for about 200 miles, to the point at which the bottom is now covered by I,700 fathoms of sea-water. Then would come the central plain, more than a thousand miles wide, the inequalities of the surface of which would be hardly perceptible, tho the depth of water upon it now varies from 10,000 to 15,000 feet; and there are places in which Mont Blanc might be sunk without showing its peak above water. Beyond this the ascent on the American side commences, and gradually leads for about 300 miles to the Newfoundland shore.
Almost the whole of the bottom of this central plain (which extends for many hundred miles in a north-and-south direction) is covered by a fine mud, which when brought to the surface dries into a grayish-white friable substance. You can write with this on a blackboard if you are so inclined; and to the eye it is quite like very soft, grayish chalk. Examined chemically, it proves to be composed almost wholly of carbonate of lime; and if you make a section of it, in the same way as that of the piece of chalk was made, and view it with the microscope, it presents innumerable Globigerinæ imbedded in a granular matrix. Thus this deep-sea mud is substantially chalk. I say substantially, because there are a good many minor differences; but as these have no bearing on the question immediately before us—which is the nature of the Globigerinæ of the chalk—it is unnecessary to speak of them.
Globigerinæ of every size, from the smallest to the largest, are associated together in the Atlantic mud, and the chambers of many are filled by a soft animal matter. This soft substance is, in fact, the remains of the creature to which the Globigerina shell, or rather skeleton, owes its existence, and which is an animal of the simplest imaginable description. It is, in fact, a mere particle of living jelly, without defined parts of any kind; without a mouth, nerves, muscles, or distinct organs, and only manifesting its vitality to ordinary observation by thrusting out and retracting from all parts of its surface long filamentous processes, which serve for arms and legs. Yet this amorphous particle, devoid of everything which in the higher animals we call organs, is capable of feeding, growing, and multiplying; of separating from the ocean the small proportion of carbonate of lime which is dissolved in sea-water; and of building up that substance into a skeleton for itself, according to a pattern which can be imitated by no other known agency.
The notion that animals can live and flourish in the sea, at the vast depths from which apparently living Globigerinæ have been brought up, does not agree very well with our usual conceptions respecting the conditions of animal life; and it is not so absolutely impossible as it might at first sight appear to be, that the Globigerinæ of the Atlantic sea bottom do not live and die where they are found.
As I have mentioned, the soundings from the great Atlantic plain are almost entirely made up of Globigerinæ, 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 siliceous bodies belong partly to the lowly vegetable organisms which are called Diatomaceæ 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 siliceous organisms, tho they are not heavier than the lightest dust, must have fallen in some cases through 15,000 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....