Botany, the science of plants, is generally divided into two branches, one of which describes their internal structure and organic action, and the other their external appearance. At the revival of learning, hardly fifteen hundred plants were known from the descriptions of the ancients. More than fifty thousand, at a reasonable estimate, have been described. Linnæus founded his system exclusively on the sexual relations of plants; dividing them all into two general divisions, one of which has, and the other has not, visible sexual parts. This division is generally adopted as the basis of elementary instruction, but many objections have been brought against it. The second general division of this science begins with the anatomy of plants, or an investigation of their internal structure. This study has been recently cultivated to a great extent, particularly by the Germans. With this division is connected chemical botany, which investigates the constituent parts, the various changes, and the different combinations of the liquid and solid parts of plants. From these we rise to the laws of vegetable life, which are generally the same with those of animal life; the physiology of plants and of animals is thus of course intimately connected. Of the two general divisions of botany, the physiological, or philosophical is the elder. It was created by Theophrastus of Eresus. Historical botany was founded by the Germans. In the seventeenth century, the foundation of botanical anatomy was laid by Grew and Malpighi; botanical chemistry was founded by Homberg, Dodart, and Mariotte: and the difference of sex was discovered by Grew, Morland and Camerarius. CHAPTER XVIII.――GEOLOGY. The first important attempt toward a scientific view of the character and relations of the strata in the United States was made by Mr. Maclure, but a short time previous to the year 1812. His work was small and general, but has proved a valuable guide to subsequent inquirers. In order to obtain a view of the general geological formation of the territory of the states, it will be well to recapitulate its chief geographical features; the Apalachian mountains on the east, with the slope to the Atlantic ocean; the Rocky mountains to the west, with the valleys intervening between them and the Pacific ocean; and the extended valley between these elevated ranges, with the Ozark mountains dividing it in the centre, and the Black mountains occupying its north-western angle.