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80 DIORITES, ETC., rally in geological age ; tlie greenstones (or diorites, &c.), being essentially hornblendic, and the basalts, augitic; and while the former have generally been irrupted in the Palaeozoic ages of geological time; the latter (with rare exceptions) are of Mesozoic and Tertiary ages. 1 We shall, therefore, consider them under two heads. Greenstones. This much used, but much abused, name is strictly confined to the group of plutonic rocks which includes cliorite, diabase, gabbro, &c., and has, therefore, only a generic signification ; its most important specific constituent being diorite. Diorite. (Diorit, Germ.) Specific gravity, 2.6-2.9; contains silica 47-58 per cent. It is a basic plutonic rock, crystalline granular to micro-crystalline, gene rally of a greenish colour, and composed of albite, oligoclase, or anorthite felspar, and hornblende, 2 and often with epidote, chlorite, pyrite, and other minerals as accessories. Occasionally mica makes its appear ance; and instances may be found where this mineral replaces the hornblende in a different part of the same rock, thus converting it into minette or mica-trap. Diorite is found amongst Silurian, Cambrian, and metamorphic rocks, generally in the form of dykes, 1 Amongst these exceptions are some of the basaltic dykes of the Mourne Mountains, in Ireland. 2 Zirkel, Petrog. i. 450. Delesse has recognised labradorite and anorthite as entering into the composition of diorite.