The Lorver Essequibo River and Cuyuni River District. 123 



sericite-schist. Colours of gold can Ije obtained from the earth on this 

 hill })j washing, whilst near by placers are being worked in alluvial 

 deposits derived from the sericite-schist. 



The rocks exposed on the series of isJaiirls in the channels between 

 which the Arawak Matope, Kumaka, and Mariwa Cataracts occur, 

 are mainly gneissose granitites passing to granitite-gneiss. 



Near the highest parts of the Portage at Arawak Matope, the 

 rocks are dark-colf)ured and compact, and consist of hornblende-schist 

 of spf^eific gravity 2-89. The lower parts of the rocks, which are dark- 

 greenish-grey in colour and have a greasy feel, show schistosity to a 

 more marked extent, the planes being in some places approximately 

 parallel to one another, in others crumpled into minute folds. The 

 rocks are traversed by thin veins of secondary ffuartz, some of which 

 show clear evidence of movement in the mass of the rock after their 

 formation being broken by small faults. Near the main fall oi the 

 cataracts portions of the rock which have not undergone metamorj^hosis 

 to the same extent as has the general mass are found among the 

 schists. These have a specific gravity of 2-82, anfl appear to have Ijeen 

 originally a feldspathic gabbro. 



The rocks in Kumaka Rapids, in the channel next to the Arawak 

 Matope ch.-mnels of the cataracts, are of gneissose-graiiitite passing into 

 more or less schistose-granitite-gneiss in the middle parts of the rapids. 



The rocks exposed in a small fall west of the main Mariwa Cataracts 

 have very well-developed schistosity, resemble slate in appearance, 

 and are greenish-grey chlorite-hornblende-schists, containing specks of 

 pyr'tes ; and weathering to a rusty-looking earthy clay. 



The main Mariwa Cataract shows a series of rocks of much interest. 

 Below the foot of the fall the rocks are fairly compact, dark olive-green 

 hornblende-epidote-schists, showing abundant small crystals and Vjlebs 

 of hornblende, and some specks of pyrite. The lowest rocks in the 

 rapids below the cataracts are pale greenish-grey sericite and chlorite- 

 schists with well-marked foliation, in places parallel, in others wavy. 

 Ascending the cataracts the rocks become more massive and, the 

 schistosity lessening, have more the character of gneiss than of schist, 

 and may be termed hornblende-granitite-gneiss. At the head of the 

 cataracts the rocks are in great rounded masses, in some of which the 

 schistosity is only marked by occasional wavy streaks of hornblende, 

 chlorite, sericite, epidote and biotite, while in other masses no signs of 

 schistosity are seen. The macroscopically unaltered rock consists of 

 granules of white and pink feldspar with intersjjersed blebs of augite 

 and chlorite, and is an augite-granitite. The rocks exposed above the 

 Mariwa Cataracts consist of a massive to a more or less gneissose- 

 granitite. The Arawak Matope, Kumaka and Mariwa Cataracts 

 furnish good examples of the changes of gabbro and granitite into 

 hornblende-schist, hornblende-granitite-gneiss, and chlorite and sericite- 

 schists. 



The rocks above Arawak Matope, extending to the Simeri Kapids, 

 consist of rather fine to medium-textured grey granitite. The Simeri 

 Rapids are over a l:)elt of very fine-grained, almost compact, dark-grey 



