124 The Geology of the Gold Fields of British Guiana. 



rock — a hornblende-mica-schist — which is traversed by intrusive veins 

 of granitite. From here to the foot of the Mutosse Rapids, a distance 

 of about a mile, the rocks are coarse-gramed granitite-gneiss, resembling 

 that seen above Mariwa Cataracts. At the foot of Mutosse is a fine- 

 grained quartz-diorite. The rocks in the middle of the rapids consist 

 of a compact micr(jgranite, passing into a rather fine-grained granitite 

 with abundant biotite. At the top of the rapids are intrusions of 

 granitite througli fine-grained mica-diorite. The structure of the rocks 

 at Mutosse is that the fine-grained diorites are intrusive in the granitite- 

 gneiss, and are themselves in turn intersected by veins of microgranite 

 and of granitite. 



The rocks from the head of Mutosse Rapids in the channels leading 

 among the islands to the south-east of Swarima Island consist of a 

 granitite of varying texture, and in the channels leading down to the 

 Mariwa Cataracts, granitite occurs of the same character as that already 

 mentioned as occurring at the upper part of the cataracts. In the 

 main north-eastern channel are some great masses of glassy-white 

 quartz, lying in a fine-grained gneissose granitite. About half a mile 

 above this, the granitite is traversed by a narrow dyke of a dark-grey 

 fine-grained ampliibolite, ^vhich is probably connected with the fine- 

 grained basic rocks seen at Mutosse Rapids in the south-western 

 channel. Above this fine and coarse-textured granitite-gneiss and 

 granitite are the prevalent rocks for about two miles. 



In the south-western channel above Mutosse, at the foot of Tak karri 

 Rapids, a fine-grained granitite is exposed, intrusive through a 

 somewhat coarse-textured gneiss with marked wavy foliation, which 

 is well seen near the middle of the rapids. At the head of the 

 rapids a dark-coloured quartz-diorite is intrusive through the gneiss. 

 On an island a little above is a large exposure of granitite-gneiss, where 

 the gradual change in the rock from an apparently massive, coarse- 

 textured granitite to a well-defined gneiss is very clearly seen. 



Above this a dyke of a greenish-grey rock of specific gravity 

 2*86, — zoisite-amphibolite — crosses the river, and is intrusive in the 

 coarse-textured gneiss, which is exposed at intervals between Tak karri 

 Rapids and the foot of Swarima Island. 



Near the lower end of Swarima Island a broad expanse of massive horn- 

 blende-granitite-gneiss is succeeded by fine-textured, and in places very 

 basic, biotite-gneiss. About a quarter of a mile ujd the channel beyond 

 the dyke of zoisite-amphibolite a broad belt of ampliibolite forms the 

 bed of the river, and extends to about 200 yards west of the Wariri 

 Mine Landing. In places the ampliibolite is altered to a talcose serpen- 

 tine rock. As this only occurs to any great extent in the higher jmrt of 

 the masses of ampliibolite, which are frequently exposed to the air, it is 

 in part, at least, a result of weathering. A little above Wariri the 

 ampliibolite gradually changes to a very coarse-textured rock made up 

 of lai-ge areas of white zoisite and patches of a pale hornblende, with 

 occasionally blebs of somewhat opalescent cjuartz. In places the 

 rock is finer in texture, but its mineralogical composition is the same. 

 The specific gravity of the coarser variety is 2-84, that of the finer 2*95. 



