IxXXviii PEOOEBDIiSrGS OF THE GEOLOGICAL SOCIETY. [May I9OI, 



Falls on the Potaro Eiver, a tributary of the Essequibo. These 

 falls, which were discovered by a Fellow of the Geological Society, 

 Mr. C. Barrington Brown, in the course of his geological recon- 

 naissance of the colony about thirty years ago, occur near the 

 escarpment of the great sandstone-formation which is so largely 

 developed in the Guianas and in Brazil. The falls are over a ledge 

 of very coarse siliceous conglomerate, some 18 or 20 feet thick, 

 which overlies a thickness of about 1000 feet of almost horizontally- 

 bedded sandstones. The river above the ; falls is about 400 feet 

 broad and from 18 to 20 feet deep, and plunges vertically, as a great 

 curtain of water, for 740 feet, into a vast chasm at the extremity 

 of a deep valley which it has eroded for a distance of about 17 miles 

 from the escarpment of the sandstones. During the first 3 or 4 

 miles of its course from the falls through the valley, the river 

 descends for about 400 feet by a series of cataracts and rapids. 

 The valley, which is eroded in places through the sandstones into 

 the underlying igneous rocks, is of surpassing beauty, and offers 

 many features of marked geological interest. One of the views, 

 taken when the water was low after a long-continued drought, 

 shows very clearly the great cave which the spray of the falling 

 water has cut out from the softer sandstone-strata. 



Others of the views show the somewhat primitive methods 

 employed in prospecting and in working the placer-claims for gold. 



With referencxj to a few rock-specimens exhibited, Prof. 

 Harrison stated that they were of diamond-drill cores from the 

 Omai-Creek claims on the Essequibo Eiver, and that they fairly 

 represented the principal auriferous rocks of that district. Omai 

 Creek is a small stream flowing into the Essequibo at about 130 miles 

 above its mouth, and the country through which it flows is usually 

 diabase (dolerite) and its decomposition-products. From a part of 

 the bed of one of the tributaries of this stream (Gilt Creek), about 

 500 feet in length by 50 in breadth, some 60,000 ounces of gold 

 and some hundreds of small diamonds have been recovered by 

 the somewhat crude methods of working hitherto in use. The 

 specimens shown were of quartz-diabase and of a massive 

 epidiorite, the oldest auriferous rocks in the district; of an in- 

 trusive aplite or possibly altered albite-granite, the con- 

 tents of which in gold (apparently of secondary origin) vary from 

 1 to 15 dwts. per ton, but in places where it is intersected by veins 

 of secondary quartz may rise to 40 or 50 ounces per ton of the 

 rock; of diabase or dolerite, so far as is at present known 

 the rock of most recent origin in the colony, and it is, in the 

 speaker's experience, invariably auriferous, appearing in fact to be 

 the principal source of gold in the Guianas; and of highly- 

 altered porphyroids, in parts changed almost completely to 

 epidote- chlorite rocks, in others to sericite-rocks wherein the 

 original porphyrites and quartz-porphy rites can be discerned only 

 with difficulty, and these in that district form the practically non- 

 auriferous country-rock. 



The so-called placer-deposits of British Guiana form a 



