22 GRANITE. pressure, and a high temperature during the process of formation. 1 Varieties of Composition. From the primary composition of granite already described, there are frequently considerable variations. Many varieties have two kinds of felspar, such as that of Egypt, which contains orthoclase and oligoclase; or that of Mourne, which contains orthoclase and albite ; others again have two varieties of mica. Accidental minerals are sometimes present, generally in druses, such as tourmaline or schorl, beryl, chlorite or talc, while hornblende often appears and sometimes replaces the mica, in which case the rock becomes a syenite. When large and distinct crystals of orthoclase felspar are individually developed, the rock assumes a por- phyritic structure, which adds much to its value and beauty for ornamental purposes. In this case, the orthose crystals often occur as twins, reflecting the light differently from their facets, and thus are capable of easy determination by the eye. These crystals frequently have a rich pink or flesh-colour, as 1 Mr. Sorby lias even gone further than this, and has shown from calculations founded on the extent to which the cavities are now filled with fluid, that certain conclusions may be arrived at regard ing the relative depths at which granite has been formed in different localities. Thus he finds that the granites of the Highlands of Scotland indicate a pressure of 26,000 feet of superincumbent rocks more than those of Cornwall; the elvans (or granitic dykes) of the Highlands, one of 28,700 feet more than those of Cornwall; but these pressures depend on the temperatures of consolidation.