The Berbice River. 177 



About half a mile above Immema-Kabura inlet is a belt of 

 extremely cuminict felsite, followed by a dyke of diabase which is 

 exposed near Kuiai Inlet. The lower edge of this diabase consists of a 

 glassy tachylyte, a form of the rock of extremely rare occmn-ence in the 

 colony. Ferruginoiis conglomerates and sandstones extend for the next 

 four and a half miles to near Dallibanna Creek. At this spot diabase 

 is seen, and extends thence for a distance of about three miles up to 

 the x\riwa Creek. 



Above the Ariwa Creek the river traverses a felsite, having a 

 tuft-like ap[)earance, indistinctly foliated, and varying in colour from 

 ashy-grey to purplish and dark red. 



Masses of this rock occur on either side of the river up to about 

 three-quarters of a mile below Deringbang Creek, where they are 

 crossed by a diabase dyke which extends a short distance beyond the 

 Deringbang Creek, forming in it, about four hundred yards from the 

 river, a cascade called Iduri-waddi. 



Between Deringbang and the foot of the Marlissa llai)ids the rocks 

 exposed at intervals are pori)hyrites and quartz-porphyry. 



At Marlissa, about thirty-seven and a half miles from Yowamia 

 Island, large rounded masses of pink-coloured aplite-granite encumber 

 the river, forming a series of narrow and shallow rapids. Many of 

 these rocks are marked with so-called Timehri pictures — ancient 

 writings or hieroglyphics — lightly cut on the surface of the granite. 



The ajilite-granite extends for about three miles to near Long 

 Liquor Rapids, where it is succeeded by feldspar-porphyrite, which 

 forms the rocky river bed, and extends a distance of about two and a 

 half miles to immediately above Itabru Creek. There two masses of 

 this rock jut out and form a narrow passage in the river, about sixty 

 feet wide, called the Itabru Gate. 



Beyond this passage the river expands, on the southern side, into a 

 bay with a sloping sand-beach, and forms a wide basin, at the extreme 

 end of which the Itabru Cataract falls through a narrow slanting- 

 channel cut through masses of feldspar-porphyrite. Another great 

 mass of this rock extends from the Itabru Cataract, in an almost 

 straight line along the northern bank of the river basin, to the Itabru 

 Gate, the weathering of which shows a globular structure, the 

 balls varying from one to three inches in diameter, being closely 

 and firmly cemented together. The masses of grey and reddish 

 feldspar-porphyrite on the south side of the cataract consist of a 

 much-jointed and compact variety breaking into squares and cuniform 

 piece.s. 



The feldspar-porphyrite extends for about half a mile beyond the 

 Itabru Cataract, about five hundred yards above which it forms the 

 Little Itabru Cataract. A small rapid, near an island about a mile 

 above Itabru Cataract, is formed by greyish rocks of quartz-porphyrite, 

 and about three-quarters of a mile above this rapid the quartz-porphyrite 

 forms two great barriers, each about eight feet high and about one 

 hundred and fifty yards apart, forming the obstructions in the river 

 knowai as the Capella and Umbrella Cataracts. 



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