CHAPTER XX, 



THE DEMERARA RIYER. 



The lower parts of the Demerara River traverse an alluvial district, 

 and no signs of rocks are seen lower than Christianburg. There the mill 

 stream cuts through a highly metamorphosed and silicified felsite or tuff, 

 resembling an indurated clay, and which contains numerous small masses 

 of quartz. J. G. Sawkins (p. 52 of the "Reports on the Geology," and 

 p. 427 of vol. xxvii. of the Quarterly Journal of the Geographical 

 Society) stated that about two miles above Christianburg a coarse- 

 grained trap rock is seen near the centre of the river at low water. 

 This I have not seen. At Three Friends and at Kumaru, near Akaima, 

 a coarse-grained diabase occurs, while Akaima Hill consists of an 

 altered felsite or tuff similar to that at Christianl)urg. There are no 

 exposures of unaltered rock l)etween Kumaru and Seba, the sections on 

 the banks of the river being generally of alluvial and flu viatile deposits, 

 although here and there granitic degradation-products occur. At Seba 

 there is a large exposure of gneissose-granitite showing porphyritic 

 crystals of feldspar, and containing caught-up masses of gneiss. The 

 granitite is traversed by many veins of aplite and of pegmatite, and 

 it forms a rounded hill of about one hundred feet altitude on the east 

 bank of the river. 



Between Seba and the Arisarabu Creek, near which is the Wallaba 

 Quarry, for a distance of about five miles the banks of the river show- 

 in places granitic decomposition-products. The rocks at Wallaba 

 Quarry consist of grey gneiss with intrusive veins of granitite and 

 of aplite. Both at Seba and at Wallaba the rocks contain small red 

 garnetSjM'hile at the latter place minute particles of pyrites, both ordinary 

 and arsenical, and of galena are present. At intervals between Wallaba 

 and Tiger Hill granitic decomposition-products and greatly decomposed 

 granite or gneiss are seen. Between three and four miles north of Malali 

 on the west bank of the river a very ferruginous sandstone, of recent 

 origin, is visible. At Surakabra, on both banks of the river, a decom- 

 posed coarse-gi-ained granitite is exposed for about three hundred yards. 

 On the eastern Hank of Tiger Hill, on the west bank of the river, great 

 masses of coarse granitite are seen in situ at intervals for about half 

 a mile ; a broad dyke of diabase penetrates them — an off-set from the 

 mass which comprises the upper part of Tiger Hill — about two hundred 

 yards from the most northerly exposure of the graiiitite. From the 

 south-easterly flank of Tiger Hill to the foot of the rapids at Malali 

 there are many exposures of diabase of medium texture. 



