The Mazaruni River. 137 



the dyke of ({uartz-diorite exposures of (luartz-diorite-gneiss occur at 

 intervals for about a mile and a half, and extend to the foot of the 

 Tupeku Cataracts. 



The Tupeku Cataracts are caused by a belt of an amphibolite and 

 hornblende-schist from three to four hundred yards wide. The general 

 strike of the foliation of the hornblende-schist varies between south 

 54" west and south 82° west, the foliation dipj^ing at high angles 

 of from 60° to about 75°, but in places the planes of foliation 

 are almost vertical, while in others they are highly contorted. The 

 dark-coloured hornblende-schist contains many thin veins of quartz 

 with grains of arsenical pyrites. In places narrow bands of amphibolite 

 are seen with but little foliation, while here and there narrow, lighter- 

 coloured bands of schist occur. Through the hcjrnblende-schist and the 

 amphibolite many intrusive tongues of pinkish granitite and of white 

 muscovite-granite are noticeable. At tlie head of the cataracts are 

 exposures of epidote-granitite-gneiss, followed by a narrow belt of 

 hornl:)lende-schist. The fact that the granitite of the district is more 

 recent than the hornblende-schist and gneiss is very clearly shown at 

 these cataracts. 



From Tupeku Cataracts to Makari Rapids the country is a gneissose 

 one. The gneiss is well seen near Poke Island and at Mary Rapids, 

 while in the channel above these rapids a narrow tongue of quartz- 

 mica-diorite traverses it. At and alDove Italjalli Rapids the gneiss 

 ex[)Osed is very contorted, and consists of bands of light-coloured 

 granitite-gneiss with dark, almost black, glistening ones of diorite- 

 gneiss. For about a mile and three quarters above the head of 

 Tamanua Hole the river flows through a plexus of islets, and is con- 

 stantly interrupted in its course by cataracts. Near the small island, 

 known as Sam Island, is a large boss of a porphyritic diorite having 

 very abundant crystals of hornblende, usualh^ from one to one and a 

 half inches across. 



Sapira Rapids occur a little above Sam Island, followed at 

 short intervals by Parawakas Cataracts, and by Paraimah Cataract 

 and Caburi or Yanissaec Cataract. The rock exposed through this 

 series of rapids and cataracts is generall}' a banded gneiss, the layers 

 consisting of granitite-gneiss, hornblende-granitite-gneiss, in places of a 

 greenish epidote-granitite-gneiss, with here and there narrow layers of 

 diorite-gneiss. Many tongues and dykes of intrusive rocks occur in 

 this neighbourhood. At Sapira Rapids the gneiss is intersected by 

 tongues of aplite, of granitite, of a quartzose felsite, and by a dyke varying 

 from eighteen to twenty-four inches across of mica-diorite. At 

 Parawakas Cataracts, where the country rock is a banded hornblende- 

 grauitite-gneiss, narrow tongues of granitite traverse the rock, which 

 is there seamed with many narrow veins of jaspery-looking quartz. The 

 Yanissaec or Caburi Cataracts are over a banded granitite and epidote- 

 grauitite gneiss, which below the main fall is traversed by a dyke, 

 trending north-north-east and south-south-west and which is about one 

 hundred and fifty feet in breadth, of a close-grained epidote augite- 

 syenite, the northern portion of the dyke being of very compact rock. 



