AN ELEMENTARY INTRODUCTION TO MINERALOGY. INTRODUCTION. 1. The surface or crust of the earth, as the very thin stratum with which we are acquainted has been named, consists of vast plains and rocky or mountain masses, the nature, causes, and relations of which are the objects of Geological inquiry. If we examine the mountain masses, we shall find, that different portions of the same rock will present great differ ences both of physical characters and chemical composition. But we may also discover in different rocks some substances which occur, either as the linings of cavities, or filling veins, or otherwise disseminated through them, and which, wherever found, present respectively nearly the same forms and physical characters, and are generally composed of nearly the same chemical constituents. These are termed minerals, and are divided into kinds or species, the descriptions of which, and the methods of dis tinguishing them from each other, are the objects of the follow ing treatise. The methods here employed are, however, occasionally de fective in consequence of differences in the physical characters of some minerals which nearly agree in their chemical constitu tion, and of differences in the chemical composition of others, the forms and other properties of which are nearly alike. B ft