. The geology of the goldfields of British Guiana. present in gneissose granites. A few grains of sphene,raiely minute crystals of zircon and some granules of iron-ore aresparsely present. Granitite.—-The varieties of granitite have been studied with thefollowing results :— (a) Grey Granitite.—-This is usually very coarse-textured, in placeswith the development of large porphyritic crystals of ortho-clase, or less commonly of oligoclase. It ranges in specificgravity from 2-61 to 2-74, according to the projwrtion ofbiotite it contains. Under the microscope it is seen to be made up of large plat
. The geology of the goldfields of British Guiana. present in gneissose granites. A few grains of sphene,raiely minute crystals of zircon and some granules of iron-ore aresparsely present. Granitite.—-The varieties of granitite have been studied with thefollowing results :— (a) Grey Granitite.—-This is usually very coarse-textured, in placeswith the development of large porphyritic crystals of ortho-clase, or less commonly of oligoclase. It ranges in specificgravity from 2-61 to 2-74, according to the projwrtion ofbiotite it contains. Under the microscope it is seen to be made up of large platesof orthoclase, some of which contain flakes of muscovite, whileothers are clouded with sericite, or are uuich of the plates show a microperthitic structure and enclosepatches or blebs of albite or of quartz. In parts large platesof microcline occur in place of orthoclase. Oligoclase occursin quantity in plates of very varying sizes, which notunfrequently contain inclusions of minute prisms of epidote ]^To face page GRANITITE. x SETTLEMENT. MAZARUNI RIVER. Fhoio by J. iriUiamt.
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