lih> CHAP. XII. On the Origin, Materials, Composition, and Analogies, of Hods. Ik it is the first error of the observer to see, like the miner, but a very limited number of rocks in the system of nature, it is not long before he falls into one the very reverse; creating for himself permanent distinctions from every incidental variety which comes under his notice. Time, however, speedily corrects this error, and teaches him, that however the aspects of rocks may be multiplied, Nature has limited these productions by a very confined set of general and constant characters. Of the Constituents of Hocks. A small number only of the Earths which Che mistry has discovered, forms the materials of all the rocks; united, in some cases, with alkalies and with certain metallic oxydes. In some, a single earth is found; in others, two or more exist; and these are either mechanically mixed, or united by the laws of chemical affinity. Thus are formed those rocks which are considered simple; simplicity, as applied to rocks, meaning simplicity of aspect. Limestone presents an example of a rock, in every respect sim ple; while basalts and clay slates, though simple as rocks, are chemical compounds or mechanical mix tures.