Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/577

 consisted of trap, trachyte with iron pyrites and red decomposed pyrites, a greenish porphyry with qnartz-veins, then trap and diorite, mica-schist, quartz, and a very hard purple trap, forming a dyke a little 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 stratification.

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.

Examination of the River 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, until 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 miles 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 hill about 350 feet high, some large boulders of granite occur.

The sand-hills seldom rise over 300 feet ; on one near Manabadin there is a landslip 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 composed entirely of sand. About two miles further up there is a hill covered chiefly with hydro-oxide of iron, bearing the appearance, in some instances, of cellular lava or scoriae from a furnace. This same formation crosses the river near Couchman's grant, where the river