THE igneous rocks of a country appeal strongly to the geologist and miner; to the former, because they are the parents of sedimentary formations and convey to him information respecting the composition and structure of the earth's crust ; to the latter, because they are so often and so intimately associated with the occurrence of ores. The study of such rocks was for a long time in an elementary state. Prior to this century geology meant mineralogy, and at the beginning of the century British geology was scarcely anything more than a prolonged battle between Neptunists and Vulcanists, the latter HUTTON'S disciples ; the former adherents of the views of WERNER, the Freiberg professor. WERNER was essentially a mineralogist, but unfortunately believed in the aqueous origin of even volcanic rocks, and -when his strange belief was completely overthrown, mineralogical science took flight to Germany, which country has been its special home ever since. WERNER considered basalt to be a precipitate from primeval oceans. His opponent Hurrox pleaded that fires beneath the surface melted granite, which protruded through previously formed strata till it reached the surface. He was most anxious to prove his theory by finding intrusive veins of granite in stratified rocks; and took a journey to the Grampians expressly to examine likely spots. When lie discovered red granite veins in Glen Tilt, branching out and penetrating limestone and other rocks, he was so transported with delight that his guides thought he had found a gold mine. He then broadly divided his rocks into stratified and igneous. But mineralogy was soon displaced in public interest by the new-born science of fossil remains. In 1815 CORDIER examined rocks by pulverising them and attempting to separate their constituents shaking table. But it was not till Dr. SORBY, 4o years ago, applied the microscope to the study of rocks in thin sections that there was a revival in the old science, this time not as mineralogy pure and simple, but as an enquiry into the origin nature, and classification of the mineral aggregates which we call rocks. The use of the microscope created a revolution in this department of geological science. Transparent slices of rock revealed their intimate structure, and a flood of light was thrown upon their origin, their consolidation and their constitution. We have good British workers in the field. JUDD, TRALL, RUTLEY, HATCH, HARKER, COLE, Will hold their own in any cosmopolitan assemblage of petrologists. Still the Continental workers dominate 'the field. FOUQUE, LACROIX, and MICHEI LEVY in France, and in Germany ROSENBUSCH and ZIRKEL are world-wide authorities on the microscopical structure of rock masses. The science is growing and just at present if we were asked to state its most striking characteristic, we should say that it is ceasing to be exclusively and merely descriptive, as hitherto it has been compelled to he, and is entering a phase in which it attempts to explain the 'way in which the molten magma of the earth's interior has differentiated into this and that description of rock.