Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/572

 a few miles south, the same characteristics appear on a larger scale in the granite, but in place of regular lines of separation a foliaceous structure predominates. At the penal settlement the land rises 56 feet ; and this is the highest point on the lower portion of the river Essequebo ; it is composed of a grey and red granite much decomposed on the surface. Some quarrying that has been done here presents good sections.

Almost immediately opposite the Commissioner's House, on the opposite side of the river, is an intrusive dyke of greenstone ; and at the mouth of the Mazuruni there is a large quarry of granite well worthy of examination.

Proceeding up the Cuyuni, instead of going over the falls of Seregatara, Tivrimi, and Ematubo, I walked across the land. The rocks observed on this path are granite, syenite, and quartz, the latter not in veins, but granulated. The same rocks occur above the cataract of Camaria and on one of the numerous islands between it and Woko Creek and Powis Hill, on which hydro- oxide of iron and haematite, with quartz in detached pieces from 2 to 6 inches in diameter, he on red and variegated clay.

At Arnakanmatabo there is an intrusive hornblendic rock, associated with granite, on the surface of which is ironstone, or hydro-oxide of iron in the form of gravel, or cemented by clay into a stiff conglomerate, a large block of which is lying at the mouth of a small stream at the east end of the Tapore Hills. On the western part of these hills there is a mass of quartz, about 200 feet in length, and 25 in width, containing minute specks of gold, lying on decomposed mica-schist, which has been mined in different directions in the expectation of intersecting it. Had it been a vein, or reef, it would have been intersected ; but it does not descend so far below the surface as where the adits are driven.

About one mile and a half up the river, there is a rivulet, where the mica-schist is more compact ; and after washing the black sand collected in the rivulet for some time I detected a few minute specks of gold, which may have been derived from the adjoining rocks, but most probably from those of the auriferous country at the head sources of the river Cuyuni, where gold was collected by the natives prior and subsequent to the discoveries of Columbus.

Proceeding up the river Cuyuni, syenite appears again at the rapids on the south of Suwaraima Island, and at the extreme north-west point of it in juxtaposition with an intrusive trap or greenstone ; these occur again about four miles further up the river.

Near the Berebisi creek and Tonoma rapids, granite and syenite occupy a considerable part of the river-bed ; they also appear at the rapids of Payuca, and up as far as the island between the Quive Kura river and the Zane Kura, at the extreme end of which there is a body of quartz, around which a quantity of black sand is deposited. In this a speck or two of gold was found; but in the quartz at this locality I could see no gold, even by the aid of a strong magnifying power.