The study of rocks, their occurrence, composition, and origin. Petrography is concerned primarily with the detailed description and classification of rocks, whereas petrology deals primarily with rock formation, or petrogenesis. Experimental petrology reproduces in the laboratory the conditions of high pressure and temperature which existed at various depths in the Earth where minerals and rocks were formed. A petrological description includes definition of the unit in which the rock occurs, its attitude and structure, its mineralogy and chemical composition, and conclusions regarding its origin. For a discussion of mineral identification, petrographic analysis, and the classification of rocks