LITHOLOGY. PART I. CHAPTER I. MINERALS. THE SEVERAL SUBSTANCES which form the materials of the earth's crust are termed Rocks,' the idea of a solid rocky substance not being necessarily implied. Most of what we call rocks are no doubt of a firm and solid character, but some consist only of soft or loose aggregates or accumulations of their component parts. These component parts are always minerals; that is to say, all rocks are mineral aggregates, consisting of minute mineral parts more or less solid and more or less intimately and firmly united, knit, or cemented together. By this definition it will be seen that we exclude the animal and vegetable kingdoms; it may therefore be well to add that under the term mineral we include all mineralised remains of organic bodies. Most rocks are made up of parts of two or more different minerals, in which case they are termed composite. Some rocks, however, consist essentially of particles of one mineral only, such for instance as limestone; these, in contradistinction to the composite, are termed simple rocks. The composite as well as the simple rocks not unfrequently contain subordinate ingredients, besides those which are essential to their character. These subordinate ingredients are termed accessory or non-essential. most cases they are inconsiderable in quantity, or they only occur locally and do not appreciably alter the nature B In |