66 The. Genhxpj of the Gold Fields of BritisJi Guiana. 



The fact tliat the massive rocks of this group changed more or less 

 gradually into schistose ones was noticed during the examination of the 

 North- Western district in 1897 ; whilst the gradual transition through 

 the stage of porphyroids of some of the varieties into schists was j^roved 

 by the field-examinations of the Essequibo-Potaro district, and the 

 laboratory studies of the rocks collected during the expeditions in the 

 years 1898 and 1899. But the origin of the rocks remained obscure 

 until they were studied in the upper parts of the Cuyuni and of the 

 Berbice Biver in 1904 and 1905. In the Cuyuni district, at Topekai 

 Rapids, near St. John's landing, at Waikuri Eapids, and near the 

 mouth of the Wenamu Creek, f eld sjoar-porphy rites and augite- 

 porphyrites were found having a more or less well-marked andesitic 

 groundmass, w^hile in the Berbice River district, at Itabru Gate and 

 Itabru Cataracts, quai'tz-porphyry occurs, showing very clearly a 

 fluxion-structure, and containing a good deal of glass in the base. At 

 Little Itabru Rapid and at Manmakuri Rapids, a f eld spar-porjjhy rite 

 of similar structure and composition was found, while at Winter's Fall 

 and at Tramway Rapid, biotite-feldspar-jjorph^'rite occurs, showing in 

 thin slices a well-marked fluxion-structure. 



Near Anaripia and Pigeon Island, in the Cuyuni River, feldspathic 

 tuffs were found, while in the Berbice River, between Ariwa and 

 Deringbang, and near Umbrella Fall, fairly well-marked tuffs occur, 

 and it is doubtful whether others of the rocks of this group collected 

 on this river are consolidated tuffs or are altered rhyolites. Much 

 altered tuffs have also been found on the Demerara and Pomeroon 

 Rivers. The porphyries and porphyrites and the schistose rocks 

 derived from them probably are therefore altered rhyolites and 

 andesites and their deeper-seated representatives. Some of them may 

 have been thin and narrow lava-flows, but the majority ajapear to be 

 portions of very widely spread thick sheets of rhyolite and andesite. 

 It is also probable that the zoisite-hornblende-schists at and near 

 Popekai Rapids in the Cuyuni River, which have been descril^ed under 

 the amphibolite and hornblende-schist group, were originally parts of 

 similar flows of basic hornblende-andesites. If this view is correct, 

 and the fleld-evidence is in favour of it, the porphyries and porphyrites 

 and their allied schists are representatives of hypabyssal and of volcanic 

 rocks varying from acidic rhyolites to basic andesites. 



