The Essrquiho River. 153 



The numerous dykes of diabase, in places changed to proterolmse or 

 to epidiorite, which traverse both the granite and gneiss from Ararapira 

 to Akenna, a distance of about nine miles, all trend towards and are off- 

 sets from the great mass of diabase which gives rise to the range of hills 

 reaching from Arasaru Hill, where it attains a height of seven hundred 

 and twenty feet, on 'the Essequibo, in a north-easterly direction to 

 Tiger Hill, and to the east of the Demerara River at Malali. This 

 great mass has evidently been thrust through a fissure or series of 

 fissures in the granite and the gneiss, which it overlies on the banks of 

 the Essequibo and of the Demerara Rivers, and has poured over them, 

 afibrding a good instance of the fissure-eruptions by which the diabase, 

 which forms an important part of the rocks of British Guiana, was 

 intruded through, over, and between the other older formations. 



At Kura Kura point the gneiss is ti'aversed by a dyke of diabase, 

 varying from forty to sixty feet in width, which trends towards the 

 north-north-west. Opposite to the Kumaparu path to the Demerara 

 River a diabase dyke near the east bank of the Essequibo River strikes 

 south-south-east and north-north-west, while about a mile south-west 

 another trends east and west, at a place known as Kumbaru Point. 

 Here the somewhat coai-se rock of the main dyke is traversed by a later 

 intrusion of very fine-grained basaltic diabase. 



At Kumbaru the river changes fi'om the south and north direction, 

 which it pursues from here to its mouth, to a west and east one for 

 about twelve miles, the middle and the north-west parts of its course 

 being very shallov,-, and covered in many places with extensive sand- 

 banks, and, beyond the turn, with rough masses of concretionary 

 ironstone conglomerate. This conglomerate common]}^ occurs in the 

 rivers of the colony where there are shallows near to extensive 

 exjjosures of diabase or of other basic rocks. 



For some distance before arriving at the German Syndicate's 

 landing at Omai, about three and a half miles west of Kumbaru Point, 

 the only rocks seen are concretionary ironstone and diabase, the latter 

 being largely exposed at the landing, where it is very coarse-grained, 

 tending in texture to a gabbro, but retaining its characteristic ophitic 

 structure. The hills in the neighbourhood are composed either of 

 diabase or of its degradation-products — ironstone and laterite. The 

 bottoms of the valleys in places, and the mining drifts and shafts near 

 Gilt Creek, penetrate into the underlying granitic and schistose rocks. 

 The road from the landing to the placer and mining claims on Gilt 

 Creek passes over the brows of hills covered with ironstone conglomer- 

 ate. On the path from the works to the falls on the Omai Creek are 

 exposures of porj^hyrite, of epidiorite, and of greatly altered diabase 

 containing much carbonate of calcium and magnesium. Great mas^-es 

 of diabase of medium texture are exposed at the Omai Falls. 



For from one and a half to two miles from the landing at Omai 

 diabase is frequently exposed in the river, and especially on its northern 

 bank, the general trend of the exposures being east-south-east and 

 west-north-west. On the southern bank of the river, about half a mile 

 west of the landing, a quartz-porphyrite of somewhat schistose structure 

 occurs, and is seen at intervals for al^out two miles. Here and there it 



