Rocks and Minerals of British Guiana. 37 



The sandstone formation constitutes a large part of 

 the Pacaraima Mountains, and crosses all the chief river 

 systems of the colony. It consists of beds of coarse 

 conglomerate, red and white sandstone, and red shale ; 

 and interbedded with these, the plutonic rock, greenstone, 

 generally occurs, partly contemporaneous and partly 

 intrusive ; and at their planes of contact the sandstone 

 usually exhibits considerable alteration. Thin beds of 

 fine red jasper, suitable for inlaid work, also occur in 

 some of the layers of this formation. 



No fossils have hitherto been found in this sandstone, 

 so that its geological classification can scarcely be con- 

 sidered as settled ; but it seems almost certainly to be 

 an equivalent of the New Red Sandstone. The most dis- 

 tinctive of the natural features of the colony, are found 

 in connection with the various conditions of this forma- 

 tion ; and denudation of a most active kind has been at 

 work during vast ages to produce conformations as 

 strange as they are magnificent, instanced best in those 

 extreme examples, the perpendicular mountain, Ror- 

 aima, on the inner confines of the colony, and the 

 unique Kaieteur waterfall on the Potaro. 



The greenstone consists chiefly of a coarse variety of 

 diorite, but its texture and its mineral character are 

 more or less varied in different places. This rock occurs 

 either in continuous layers over a large extent of coun- 

 try, or as isolated hills evidently formed by denudation, 

 or as extensive dykes piercing through granite, gneiss, 

 quartz-porphyry and sandstone, over the whole surface 

 of the colony, and more especially noticeable alono - 

 the river courses in the dry seasons. Igneous action 

 must have been, at the date of eruption, extremely 



