422 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [May 24, 



a few miles south, tlie same characteristics appear on a larger scale 

 in the granite, hut in place of regular lines of separation a foUa- 

 ceous 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 de- 

 composed 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, lie on red and variegated clay. 



At Arnakanmatabo there is an intrusive hornblendic rock, asso- 

 ciated 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 de- 

 composed 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 de- 

 posited. In this a speck or two of gold was found; but ia the quartz 

 at this locality I could see no gold, even by the aid of a strong mag- 

 nifying power. 



