168 Thp. Geology of the Gold Fields of British Guiana. \ 



The valley of the Minnehaha has been eroded through a district 

 where quartz-porphyry, porphyrite, and similar rocks were intruded 



into and overlaid by great masses of diabase ; and the deposits in it I 



represent the degradation-products of these rocks with some sandstone ; 



quartz-sand, and pebbles derived from the sandstone formation which j 



is found to the south and west of the district, and which, probably I 



once extended over it, 'J' , 



