petrology

[Gr. petros =a stone, a rock; suff.- ology.] Nat. Science : The study of the mineralogical and chemical composition of rocks; including the various changes they have undergone through phys ical and chemical agencies, either combined or sep arate. Macroscopic and microscopic examination, together with chemical analysis, are the methods pursued. If the rock is of sufficiently coarse texture, an examination by an ordinary lens suffices, but in rocks of fine grain a thin section is prepared, and, under the microscope, the individual mineral constituents are recognized by their specific structural and optical characters. The chemical analy sis is performed either on the rock as a whole, or the ma is pulverized, and the mineral species of which it consists separated by certain fluid chem ical compounds of known density, and the products of this separation separately analyzed.