These statements are not, as the reader might conceive from their nature, a romantic conception invented to please the imagination of those in search of a system--the time is past when geology should be regarded as the romance of Nature--nor has what we advance at all the character of an arbitrary conception. One is no doubt struck with surprise on learning, for the first time, that all the limestone rocks, all the calcareous stones employed in the construction of our dwellings, our cities, our castles and cathedrals, were deposited in the seas of an earlier world, and are only composed of an aggregation of shells of Mollusca, or fragments of the testaceous coverings of Foraminifera and other Zoophytes--nay, that they were secreted from the water itself, and then assimilated by these minute creatures, and that this would appear to have been the great object of their creation in such myriads. Whoever will take the trouble to observe, and reflect on what he observes, will find all his doubts vanish. If chalk be examined with a microscope, it will be found to be composed of the remains of numerous Zoophytes, of minute and divers kinds of shells, and, above all, of Foraminifera, so small that their very minuteness seems to have rendered them indestructible. A hundred and fifty of these small beings placed end to end, in a line, will only occupy the space of about one-twelfth part of an inch. [Illustration: Chalk under the Microscope. Fig. 126.--Chalk of Meudon (magnified).] Much of this curious information was unknown, or at least only suspected, when Ehrenberg began his microscopical investigations. From small samples of chalk reduced to powder, placed upon the object-glass, and examined under the microscope, Ehrenberg prepared the designs which we reproduce from his learned micrographical work, in which some of the elegant forms discovered in the Chalk are illustrated, greatly magnified. Fig. 126 represents the chalk of Meudon, in France, in which ammonite-like forms of Foraminifera and others, equally beautiful, appear. Fig. 127, from the chalk of Gravesend, contains similar objects. Fig. 128 is an example of chalk from the island of Moën, in Denmark; and Fig. 129, that which is found in the Tertiary rocks of Cattolica, in Sicily. In all these the shells of Ammonites appear, with clusters of round Foraminifera and other Zoophytes. In two of these engravings (Figs. 126 and 128), the chalk is represented in two modes--in the upper half, by transparency or transmitted light; in the lower half, the mass is exhibited by superficial or reflected light. [Illustration: Chalk under the Microscope. Fig. 127.--Chalk of Gravesend. (After Ehrenberg).--Magnified.]