134 The Geologi/ of the Gold Fidxh of British Guiana. 



To the south of Echilebar the country consists of (juartz-porphyry, 

 gneiss, granites and gabbro, and is traversed in places by dykes and 

 sills of diabase. 



Between the Upper Potaro and the Ireng Rivers, and along the course 

 of the Kopinang Rivei-, there are very extensive elevated flats, and, in 

 places, mountains which ai'e covered by concretionary ironstones, the 

 country consisting of diabase. This rock thei'e forms widely extending 

 sheets and sills of great thickness. It is exposed along the southern 

 banks of the Kopinang River for a distance of about thirty miles. 



Extensive beds of coarse conglomerates occur in parts of the sand- 

 stone district, as near the sources of the Wenamu River, and in the 

 Merume, Kaieteur and Tawailing districts. 



The sandstones in many places show a case-hardening to various 

 depths by the deposition of quartz around their grains. The silica has 

 doubtless been derived in part from the decomposition of the felds- 

 pathic constituents of the sandstones, but may also have been derived 

 from the slow solution of some of the quartz grains. The case- 

 hardening of rocks by the deposition of silica, which has been dissolved 

 in rain-waters percolating through them, in the form of quartz, is not 

 an uncommon occurrence in tropical regions, where during dry seasons 

 evaporation from the upper parts and the surfaces of rocks takes place 

 with great rapidity. The Kaieteur Plateau is an excellent example of 

 this. The flattish rounded pebbles of white quartz which form the 

 mass of the conglomerate are more or less etched on their upper 

 surfaces where exjDOsed on its bare flats, whilst they are so thoroughly 

 cemented ])y secondary quartz in their matrix that it is frequently 

 easier to break them across than to detach them from it. 



There is a very remarkable development of this action in many 

 places on the plateau which forms the top of Mount Roraima. The 

 surfaces of shallow l^asin-like depressions on the summit of the 

 mountain, which during wet seasons contain accumulations of rain- 

 water, are covered with more or less horizontal masses of crystalline 

 quartz, from which project into the water very numerous prisms of 

 clear colourless or occasionally slightly clouded quartz, some of which 

 are nearly one and a half inches in length. 



