24 Geohxjij of tJic. (jold Ficldx of Briti>ih Guiana. 



able to examine them, show no signs of having, even in part, been 

 derived from later rocks. If it is of Cretuceous age it offers an 

 interesting example of the recurrence of similiar formations in widely- 

 divided geological ages, when the conditions all'ecting their formation 

 and deposition are identical. Personally I am not j^repared 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, por23hyries, felstones and schists derived from them. 



The sandstone formation spreads eastwai'dly 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 con- 

 glomerate, red and white sandstones of very varying texture, and in 

 places of strata of red shale. 



High mountains occur in the sandstone formation, which consist of 

 coarse-textured diabase or of rather fine-grained gabbro. This rock 

 shows signs of metamorphism, in places being granulitic in structure 

 and in others being changed to a considerable extent, either by the 

 development in it from augite of a dark-brown secondary biotite, or the 

 pyroxene is altered fi-om an almost colourless mineral to a brown- 

 coloured strongly dichroic one. 



Mr. C. Wilgress Anderson, who in 1895 spent several months in 

 traversing the sandstone district while inquiring into the alle<.'ed 

 occurrence of beds of auriferous conglomerate in it, and has siuce 

 crossed it repeatedly during the Roundary Surveys, is of opinion that 

 the diabase-gabbro is of greater age than the sandstone, the latter 

 formation in places resting on or abutting against it, and this view is 

 upheld by its structure. The hills were probably small islands in the 

 shallow seas in which the sandstone formation was laid down. C. B. 

 Brown, on page 14 of the " Reports on the Geology of British Guiana," 

 mentions the occurrence of great layers of conglomerate in the 

 neighbourhood of " greenstone," and this is confirmatory of Mr. 

 Anderson's view. The possible existence of " greenstone " of two 

 distinct geological ages and modes of occurrence does not s^em to have 

 struck Brown and Sawkins, but it offers an intelligible explanation of 

 the facts recorded by^ them in their reports. These surveyors estimated 

 the total thickness of tlie sandstone on the assumption tliat it is 

 traversed by three layers of greenstone at about three th(»usand feet. 

 As, however, it is probable that some at least of the latter diabase, as, 

 for instance, that at Roraima, is in the form of laccolites, and during 

 intrusion lias elevated great tracts of the sandstone country, probably 

 the formation has not the total thickness deducible from C. B. Brown's 

 figures, and may at jjresent not anywhere exceed in thickness that shown 

 at Roraima — about two thousand feet. As a rule the sandstone lies 

 nearly horizontally, dipping somewhat to the north, and few faults are 

 seen in it although in j)laces near where diabase has intruded into it 



