EIGENMANN: THE FRESHWATER FISHES OF BRITISH GUIANA 13 



as to this. C. B. Brown arrived at the conclusion on what appears to me to be 

 somewhat defective evidence (its, in parts, reddish color, its unfossiliferous nature, 

 and its being penetrated by masses, dykes, and sills of greenstone — diabase — as 

 are sandstones of Triassic age in North America), that it is an equivalent of the 

 New Red sandstone. In Venezuela its relationship to rocks of known age is said 

 to be recognizable, and it is stated to be of Cretaceous age. A like conclusion that 

 the northern parts of the format'on are of Cretaceous age has been arrived at in 

 Brazil. If these views are correct, the later outbreaks of diabase, which are, directly 

 or indirectly, the causation of many of the auriferous deposits of British Guiana, 

 must be either of Cretaceous age, or belong to the Tertiary or to a later period. 

 And as there is a very great resemblance in the magmatic character of the Guianan 

 diabase, and of the lavas of the West Indian province, whose outbreaks are clearly 

 of Tertiary and of present age, the assumption of the relatively recent age of the 

 diabase is a plausible one. As will b? mentioned in a later chapter, the diabase 

 shows no signs of the effects of the regional metamorphism which has materially 

 affected many of the rocks underlying the sandstone formation. 



"The only evidence available in this colony with regard to the sandstone and 

 the geological period of its formation is that wherever its base has been seen it 

 occupies an analogous position to the Torridonian sandstones of the Scottish High- 

 lands, to which the sandstone has a close resemblance in constitution. It lies 

 invariably on the presumably Archean rocks of the colony; and its constituents, 

 as far as I have been [p. 24] able to examine them, show no signs of having, even in 

 part, been derived from later rocks. If it is of Cretaceous age it offers an interesting 

 example of the recurrence of similar formations in widely divided geological ages, 

 when the conditions affecting their formation and deposition are identical. Per- 

 sonally I am not prepared from my own observations and studies to accept any 

 statements of its geological age further than that shown by its relationship to the 

 underlying gneiss, porphyries, felsite and schists derived from them. 



"The sandstone formation spreads eastwardly through the colony, crosses the 

 Essequibo River in a low narrow belt at Comuti Mountain, gives rise to the Maccari 

 Mountain in Demerara, and crossing the Berbice River near Marlissa Rapids, is 

 seen forming a low mountain range at Itabru near that river. It passes into Dutch 

 Guiana across the Courantyne River near its union with the Cabelebo River, and 

 also in its higher reaches. The formation consists of beds of coarse conglomerate, 

 red and white sandstones of very varying textures, and in places of strata of red 

 shale. 



"High mountains occur in the sandstone formation, which consist of coarse 



