Now, geologists find that some limestone formations met with in the stratified rocks have certainly been formed in this way; for example, certain parts of the great "mountain limestone." This is proved by the fossil corals it contains, and by tracing the old coral reefs; but it is also largely formed by the remains of other graceful calcareous creatures known as encrinites, or "sea-lilies," with long branching arms that waved in the clear water. Such creatures still exist in some deeper parts of the sea, and look more like plants than animals. In former ages they existed in great abundance, and so played an important part as rock-formers,--for their stems, branches, and all are made of little plates of carbonate of lime, beautifully fitting together like the separate bones, or vertebræ, composing the backbone of a fish; and when the creatures died, these little plates no longer held together, but were scattered on the floor of the sea-bed. Shell-fish abounded too, and their shelly remains accumulated into regular shell-beds in some places. But at times mud and sand would come and cover over all these organic deposits. But of all rocks that have an organic origin, chalk is the most interesting. Geologists were for a long time puzzled to know how this rock could have been formed; but some soundings made in the Atlantic Ocean previous to the laying of the first Atlantic cable led to a very important discovery, which at once threw a flood of light on the question. Samples of the mud lying on the bed of this ocean at considerable distances from the European and American coasts, and at depths varying from one thousand to three thousand fathoms, were brought up by sounding apparatus. Little was it thought that the dull grey ooze covering a large part of the Atlantic bed would bring a message from the depths of the sea, and furnish the answer to a great geological problem. Yet such was the case; for under the microscope this mud was seen to be chiefly composed of very minute and very beautiful shells, now known as _foraminifera_, and much prized by microscopists. These tiny shells are found at or near the surface of the sea; and after the death of the creatures that inhabit them (which are only lumps of protoplasm with no organs of any kind), the shells slowly sink down to the bed of the ocean. Now, these creatures multiply at so inconceivable a rate that a continuous shower of dead shells seems to be taking place, and the result is the slow accumulation over vast areas of the Atlantic and Pacific oceans of a great deposit of calcareous ooze, which if raised above the sea-level would harden into a rock very similar to chalk. [Illustration: MICROPHOTOGRAPHS ILLUSTRATING ROCK FORMATION. I. Foraminifera. II. Section of Granite. III. Nummulitic Limestone.]