126 The Geology of the Gold Fields of British Guiana. 



quartz, and containing a vein of dark-grey quartz with specks of 

 pyrite. 



From Maritout to Stop-Off Rapids the countiy traversed by the 

 river consists of somewhat coarse-textured granitite-gneiss with, in 

 places, bands of diorite-gneiss. At Nosan Island the granitite-gneiss 

 has a granitic character, and in hand-specimens has the appearance 

 of a massive granite. Stop-Off Rapids are over a dyke of medium- 

 textured olivine-diabase which strikes through the gneiss east- 

 north-east and west-south-west, the dyke there being about 120 feet 

 wide. 



The eastern channel above Akariwa Cataracts traverses similar 

 country — gneissose-granitite and granitite-gneiss — to that of the western. 

 As a local modification about 2 miles above the cataracts a narrow 

 band of muscovite-gneiss occurs. 



About half a mile above the northern end of Swarima Island the 

 north-east part of the dyke of olivine-diabase which, at the head of the 

 western channel, gives rise to the Stop-Off Rapids, is exposed. The 

 dyke is there about 50 feet across. 



The Stop-Off Dyke is of considerable interest. It trends east-north- 

 east to west-south-west, and is exposed, when the water is low, for nearly 

 a mile across the river. It is broader in its western end than in its 

 eastern, and appears to die out near the eastern bank of the river. It is 

 composed of a somewhat coarse olivine-diabase of specific gravity 3*00. 

 Olivine-diabase is a rock of relatively rare occurrence in the districts of 

 the colony recently examined. 



The river between Stop-Off Rapids and Tinamu Cataracts, which 

 are situated about 3 miles to the north of the former, traverses a 

 district of granitite-gneiss and gneissose-granitite. Tinamu Cataracts 

 are caused by a diabase-dyke from 130 to 150 feet in breadth trending 

 east-north-east and west-south-west. The rock of the dyke is fine- 

 textured at its margins and somewhat coarser-textured towards the 

 middle, where, in places, the diabase contains a little olivine. 



The granitite-gneiss near the dyke has been much altered, the 

 original ferro-magnesian minerals, in specimens taken from near the 

 actual contact, being completely destroyed, and represented only by 

 patches of minute grains of magnetite and of secondary augite, whilst 

 the rock has assumed a granitic structure. 



For about 2^ miles above Tinamu Cataracts the river flows fi'om the 

 west-south-west, and its right bank is l^elow a low range of diabase hills 

 which extends from Tinamu Cataracts to Tagina Point. The altered 

 granitite-gneiss is exposed in places near the right bank ; whilst at the 

 mouth of the Moco-Moco Creek, about 1 ^ miles south-west of Tinamu, 

 diabase and altered gneiss are seen close together, although the actual 

 contact is hidden. The granitite-gneiss is exposed at the mouth of the 

 creek, and rather coarse-textured diabase a few yards up it. The former 

 rock here has a micro-peg matitic structure, and may be regarded as an 

 altered granophyre. A few yards above Moco-Moco Creek a fine-grained 

 variety of diabase occurs in the river, and near it are rocks consisting of 

 profoundly altered granitite-gneiss. 



