. The geology of the goldfields of British Guiana. k of the Essequibo about half a mile from theend of the island. At the mouth of the Potaro River the Essequibo is crossed from eastto west by rocks having a marked schistose structure, and weatheringinto great slabs. Those exposed on the east bank are much altered byweathering and are practically quartz-schist, while those in the middleof the river, at the east and west ends of the island in the mouth of thePotaro and on the south l)ank of the latter river, near its mouth, aresericite-schists derived from quartz-i:)orphyrite. The schist can be
. The geology of the goldfields of British Guiana. k of the Essequibo about half a mile from theend of the island. At the mouth of the Potaro River the Essequibo is crossed from eastto west by rocks having a marked schistose structure, and weatheringinto great slabs. Those exposed on the east bank are much altered byweathering and are practically quartz-schist, while those in the middleof the river, at the east and west ends of the island in the mouth of thePotaro and on the south l)ank of the latter river, near its mouth, aresericite-schists derived from quartz-i:)orphyrite. The schist can beti-aced to the quartz-porphyrite from which it has been derived andwhich is seen in places almost unaltered. A highly metamorphosed rockoccurs on the west bank of the Essequilio, south of the Potaro mouth,in the form of a fine-grained chloritoid rock with a greasy feelsomewhat resembling that of serpentine. In this schistosity has beendeveloped, but not the same extent as in the xocks derived hornquartz-porphyrite. To face ))(i(/e 155.] Plate DIABASE ROCKS, WITH WlilTIXOS, AT WARAPUTA CATARACT, ESSEQUIBO RIVER. P/iofo hn 11. I. TJifi Essequibo River. 155 On the west bank of the Essequibo River, about one and a half milesfrom the mouth of the Potaro and north of Yesi Island, are largeexposures of quartz-porphyrite, whilst similar ones occur on the east bankopposite to the island. Rounded masses of fine-grained gneiss, muchaltered by weathering, occur on the east bank of the river near thesouth of Yesi Island, and are traversed by a dyke of compact, dark-greyquartz-porphyrite which weathers into angular blocks. The dyke givesrise to small rapids in the eastern channel of the river. South of YesiIsland the rocks have a more or less marked schistose structure, andare derived from porphyrites, felsites, and quartz-porphyrites. About a mile north of the mouth of the Konawaruk Creek a narrowdyke of diabase crosses the river from east to west, and half a mile so
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