. The geology of the goldfields of British Guiana. instead of in large irregular ophiticmasses of augite. In some of these rocks porphyrinic crystals of augiteor of labradorite have developed, and they might be termed augite-porphyrite. The majority of the narrow columnar dykes or tonguesare best described as basalts of the tholeite type. In colour the diabase varies from dark-green to greenish-grey, grey,and dark-grey; it is most usually of a very dark stone-grey is heavy, and, as a rule, dense in structure. Its clean-fracturedsurface is dull and almost without lustre. The rock is h
. The geology of the goldfields of British Guiana. instead of in large irregular ophiticmasses of augite. In some of these rocks porphyrinic crystals of augiteor of labradorite have developed, and they might be termed augite-porphyrite. The majority of the narrow columnar dykes or tonguesare best described as basalts of the tholeite type. In colour the diabase varies from dark-green to greenish-grey, grey,and dark-grey; it is most usually of a very dark stone-grey is heavy, and, as a rule, dense in structure. Its clean-fracturedsurface is dull and almost without lustre. The rock is hard, verytough, and resistant to the hammer ; and blocks of it, when struck,resound with a clear ringing tone. The weathered surface of themasses of the rock exposed in cataracts or rapids in the courses of thei-ivers are covered with a thin coating or skin of varying shades ofbrown Iesulting from the oxidation of the iron-bearing minerals; andwhere its surfaces are not exposed to the scour of rapidly running Plate 8. [To face page 86. f - ^-r:^. TUMATUMARI CATARACT, POTARO RIVER. PORTIOX SHOWING EXPOSED DIABASE KOCKS. Photo hy C. H. Anderson. Tim Petrography of the, Intrusive Diabase. 87 waters they are frequently covered with very rough layers, from one-half to two or three inches in depth, of concretionary ironstone. Thecoarse-textured varieties weather with a very rough surface, occasionedby the masses of ophitic augite with their contained crystals offeldspar being far more resistant to the action of rainwater than arethe intervening patches of lime-soda feldspar, which weather away,leaving the former standing out from the surface of the rock. Not unfrequently by the sides of the rivers and streams, inravines, and in places in the heart of the forest, masses of this rock areseen showing deep rounded flutings on their sides. These are verycharacteristic of the diabase, but it is difficult to explain their origin,the most feasible explanation as yet adduced being that they h
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