Whilst Geology was still in its infancy, more attention was paid to the broader or more general features of the earth’s structure and past history, than to the minuter details of its component parts. The arrangement of the rocks forming that portion of its crust accessible to man, their organic contents as affording a key to their relative Chronology, the physical causes producing the various changes that are seen to have taken place in connection with these rocks and their enclosed fossils, the chemical composition of the different minerals, these were amongst the principal objects set before the Geological student, and are those which still largely engage his attention; but of late years we have been taught to see that would we thoroughly comprehend the history of the rocks of our globe, we must not only study them on a large scale, not only analyse them in the chemical laboratory, but we must not rest satisfied until we have searched into their innermost structure, and have learned to unravel by means of the microscope and the polariscope the wondrous history of their formation, and of the varied changes that they have passed through during the lapse of ages. There are many questions which cannot be answered by the unaided eye. We can, indeed, by the eye alone learn to distinguish between most of the ordinarily met with igneous and aqueous rocks. A mere tyro in geology sees at a glance the difference between grits, sandstones, limestones, shales or slates, and ...