1871.] SAWKINS BRITISH GUIANA. 4i}7 



consisted of trap, trachyte witli iron pyrites and red decomposed 

 pyrites, a greenish porphyry with qxiartz-veius, then trap and 

 diorite, mica-schist, quartz, and a very hard purple trap, form- 

 ing a dyke a Kttle below the great falls of Piamah, which are now 

 cutting away beds of sandstone and conglomerate that present 

 escarpments like masonwork, so regular are the lines of stratifi- 

 cation. 



This was the extent of my second excursion and examination on 

 the Mazuruni river. I estimated the distance from the coast to be 

 430 miles. 



EoGcimination of the Kiver Demerara. 



The lower portion of this river is exceedingly monotonous and 

 uninteresting, being formed of low alluvium from the coast up to 

 the "sandhills." Between these two points there is little else than 

 swamp or low muddy flats covered by caladium, arborescent arums, 

 mangroves, &c., and affording not even a landing-place of sufficient 

 firmness to support a man. 



The sand-hills are low and undulating, composed chiefly of a 

 whitish sand with a little clay ; they rise a little as they recede from 

 the river, but nowhere exceed the height of 70 feet, untU we arrive 

 at Berlin. 



At Dalgnin the hills rise higher, but none so much as 200 feet, 

 until near Christianburg, where the country becomes decidedly 

 hilly. About two mUes above this the first rock, a coarse-grained 

 trap, is seen near the centre of the river at low tide. 



At Akyma one of these hills has been cleared of timber, and gives 

 an opportunity of seeing the configuration of the surrounding 

 country. To the east and south the land is low and swampy, and 

 is often overflowed by the river. 



At Seba a boss of granite (common) rises from 15 to 18 feet on the 

 east side of the river, exposing an area of more than fifty yards ; and 

 at Tiger' s-leap, on a hiU about 350 feet high, some large boulders of 

 granite occur. 



The sand-hiUs seldom rise over 300 feet ; on one near Manabadin 

 there is a landshp or breakaway, by which the lines of stratification 

 are seen resting on granite. 



At the rapids of Mouranicrocaba the granitoid schist constitutes 

 the barrier across the river, forming rapids and falls of 5 or 6 feet 

 perpendicular. 



At Labacabra granite appears, and continues up the river as far as 

 Curuwa creek, where a fine-grained hornblende rock occurs ; the 

 same appears at Curabelicabra, also at Morocabra, and occasionally as 

 high as Arampa, where sand and clay compose the banks. At the 

 base of a hill near Umpa a white pottery-clay forms a bold bank. 



At the north of the next creek, Cornbara, there is an island com- 

 posed entirely of sand. About two miles further up there is a hill 

 covered chiefly with hydro-oxide of iron, bearing the appeara.nce, in 

 some instances, of cellular lava or scorise from a furnace. This same 

 formation crosses the river near Couchman's grant, where the river 



