. The geology of the goldfields of British Guiana. e rock gradually changesinto a fairly fine-grained one, where it is last seen at the end of theKumaparu path. From near the end of the Kumaparu path the river resumesits southerly course, and its banks indicate that it traverses agneissose district. About a mile north of tlie great falls of OroruMalali coarse hornblende-granitite-gneiss is exposed on the east bankand at intervals from there to the foot of the falls. The falls arecaused by a great Vjelt of diabase of very coarse texture, which cutsthrough the gneissose rocks in an easterly and
. The geology of the goldfields of British Guiana. e rock gradually changesinto a fairly fine-grained one, where it is last seen at the end of theKumaparu path. From near the end of the Kumaparu path the river resumesits southerly course, and its banks indicate that it traverses agneissose district. About a mile north of tlie great falls of OroruMalali coarse hornblende-granitite-gneiss is exposed on the east bankand at intervals from there to the foot of the falls. The falls arecaused by a great Vjelt of diabase of very coarse texture, which cutsthrough the gneissose rocks in an easterly and westerly direction. Theeffects of the intrusive mass are distinctly noticeable on the gneissat about fifty yards from the contact, and are very markednear it, where the gneiss assumes a granitic structure, thefoliation gradually becoming obliterated. At the falls near the diabasethe gneiss resembles a porphyritic granitite with crystals of orthoclaseand oligoclase, with rounded outlines surrounded by hornblende and [_Tii fitce jwge 171. Plate ORURO MARALl OR GREAT FALL, OVER DIABASE,DEMERARA RIVER. Fhofo Inj II. I. rcrlun The Demerara River. ] 75 chlorite after biotite. Above the falls the diabase has a coarselygranular structure, and has weathered into rounded its contact with the gneiss its texture becomes finer, the ro3kweathering into angular blocks ; and it gradually changes to a veryfine-grained basaltic diabase, which in the places of actual contact is avery compact dense proterobase showing a fluxion structure. Theintrusive diabase mass sends out many narrow veins, which split upand follow lines of weakness in the gneiss. In these the diabase isconverted into an epidiorite. The country in the neighbourhood of the Kanaimapoo, Appaparuand Darina mines consists either of epidiorite, of epidote-hornl^lende-schist or of quartz-schist and chlorite-schist. It is intruded into bynumerous dykes of diabase which give rise to ranges of low veins have
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