14 MEMOIRS OF THE CARNEGIE MUSEUM 



textured diabase or of rather fine-grained gabbro. This rock shows signs of meta- 

 morphism, in places being granuhtic 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 from an almost colorless mineral to a 

 brown-colored strongly dichroic one. 



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

 the sandstone district while nquiring into the alleged occurrence of beds of aurifer- 

 ous conglomerate in it, and has since crossed it repeatedly during the Boundary 

 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 wer^ 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 neighborhood 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 seem 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 the sandstone on 

 the assumption that it is traversed by three layers of greenstone at about three 

 thousand 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 

 has 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 present 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 places near where diabase has intruded 

 into it [p. 25] there are well-marked local disturbances in its dip. Many of the 

 beds of sandstone of finer texture show well-marked current-bedding." 



