148 Tlie Gooloijy of tin' Gold Fleldi^ of liriiiah Guiana. 



occur with, in places, the rock weatliered into great slabs. As a rule, 

 the foliation of the gneiss is well-marked, the general trend of the 

 lamination being soutli-west and north-east, but in places being in the 

 directions of either west-north-west and east-south-east or of east-north- 

 east and west-south-west. At Taparau Island and landing the rocks 

 exposed are medium to fine-grained granitite-gneiss, with layers of 

 hornljlende-granitite of fairly coarse texture. Between Taparau and 

 Thomas Cataracts the rocks are granitite-gneiss and hornblende- 

 granitite-gneiss of varying texture. A little below Thomas Cataract, 

 on the left bank of the river, a bank about 30 feet in height of red clay 

 is seen, probably a decomposition-product either from diabase or from 

 some other basic rock, followed by exposures of concretionary ironstone. 



Thomas Cataract is caused by a belt of somewhat gneissose pinkish 

 hornblende-granitite which extends for some 600 yards al)Ove the 

 cataract. For about a mile above the belt of granitite near Thomas 

 Cataract gneiss is exposed, and it is followed by a narrow band of 

 granitite, probably about a quarter of a mile across, through which a 

 dyke of diabase trends north 20" west and south 20° east, its breadth 

 being from 12 to 15 feet. From above this belt of granitite to the foot 

 of the Great Falls the rocks are gneiss, and in places the closeness of 

 the foliation causes them to resemble crystalline schists. The small 

 cataract called Great Falls is caused by a belt of reddish gneissose 

 hornblende-granitite, about a mile in width, which extends from its foot 

 to above the foot of the itabo near the head of Long Falls. Above the 

 granitite is a long series of exposures of epidiorite, of amphibolite, and 

 of hornblende-schist. The first exposures are of an epidiorite, which 

 originally contained large phenocrysts of augite, now altered to horn- 

 blende, and are succeeded by some of a more or less schistose amphi- 

 bolite, which near the mouth of Tiger Creek and for some 250 yards 

 aljove, shows the structure of horneblende-schist, the strike of its 

 foliation being about north 20" west and south 20° east. 



About 300 yards to the north of Tiger Creek the hornblende-schist 

 is traversed by a narrow belt of very fine-grained biotite-schist. At 

 the foot of the Stop Falls Rapids the rock is an epidiorite, while 

 the middle and upper parts of the rapids are over fine-grained 

 actinolite-schist, extending for about three-quarters of a mile along the 

 course of the river above Stop Falls. Here the river traverses a belt of 

 hornblende-granitite-gneiss for about a quarter of a mile, while thence 

 to Williams and Essex landing the rocks are of finer grain than those 

 below Stop Falls, and, as a rule, consist of feldspathic hornblende-scliist, 

 with, in places, belts and masses of coarser-grained more basic 

 ampliibolite. From Williams and Essex landing to Burgomaster 

 landing the i-ocks seen are hornblende-schist and amphibolite. 



The belt of epidiorite, amphibolite and hornblende - schist 

 traversed by the river has a breadth of at least 4 miles from east 

 to west and a length of about 7 miles from north to south, but its area 

 is probably far greater. Through the belt, in addition to the unaltered 

 rocks seen, concretionary ironstone in large boulders, and in places 

 forming rock-masses of considerable extent, is of frequent occurrence, 



