140 Tlie Geohujy of the Gold Fields of Britifth Guiana. 



fine texture and of a lighter shade than that of the main dyke. 

 Frecjuent exposures of gneiss occur between Kural)iri and above the 

 head of the Kabuwira Cataracts, the rock being well seen*near the 

 Pacapoey Rapids. At Kabuwira the gneiss in the lower and upper 

 cataracts is very feldspathic with little mica, while that in the middle 

 parts is dark-coloured granitite-gneiss showing Tvell-marked foliation. 



A little distance above the head of Kabuwira several dykes of 

 diabase occur ; the first one being forty feet in breadth and striking 

 north-east and south-west, a second, a narrower one, trending east and 

 west, and a third traversing the gneiss in the direction of east-south- 

 east and west-north-west. About one hundred yards higher up the 

 river a dyke of the same rock strikes north 60° west and south 60" east, 

 and another about sixty feet across trends to the south-south-west and 

 north-north-east. Near the foot of Itaballi Rapids, about a quarter of 

 a mile from the head of Kabuwira, a diabase-dyke runs in a direction 

 somewhat north of east through granitite-gneiss, and gives rise to a small 

 island, while at the head of the rapids a dyke of diabase about sixty 

 feet in width strikes north-east and south-west, and after passing 

 through a small island splits into two dykes ; above here diabase is 

 exposed at intervals on three small islands, and apparently gives rise 

 to a small rounded hill on the mainland near the right bank of the 

 river. 



About half a mile b>^yond the head of Itaballi Rapids great masses 

 of glassy, white quartz are exposed in the gneiss. The samples 

 collected yielded gold upon assay at the rate of seven grains per ton of 

 the rock. About three hundred yards above the masses of quartz a 

 diabase-dyke strikes south-north-west and north-north-east. A little 

 above a reef of white, sugary quartz, seen for a length of about twenty 

 feet, runs north 80° west, is about five feet in thickness, and dips 

 at an angle of nearly 70° through gneiss. Samples yielded upon assay 

 nine and a half pennyweights of gold iper ton of quartz. Somewhat to 

 the east of this two smaller exposures of quartz occur which probably 

 are portions of this reef. 



Kartauari Cataracts are about five miles higher up the river. 

 The river flows with great velocity through the channels below 

 them, and is, in places, interrupted by small rapids. For the 

 first two miles of the channel the rocks exposed consist of graiiitite- 

 gneiss and of coarse-textured, almost granitoidal, hornblende- 

 granitite-gneiss, the latter being traversed, near a series of 

 small rapids, by a narrow vein of diabase. The hornblende- granitite- 

 gneiss is of a reddish colour, and weathers either into rough jagged 

 masses or into large pointed pinnacles. Below Kartauari Cataracts 

 the gneiss is a fine-grained granitite-gneiss, with well-marked foliation, 

 ajid is traversed by a broad belt of augite-granitite or (juartz- 

 monzonite. This has given rise to many great masses of rock 

 weathered into exti-aordinary shapes, several of them having striking 

 resemblance to gigantic crouching toads. Of these, the largest and 

 most remarkable is the one situated near the right bank of the main 

 channel, called the Kartauari Rock. This rock and the rounded 



