1871.] SAAYKIXS BRITISH GUIANA. 433 



feet from the summit to the iuclined portion surrounding it like a 

 glacis or closely built buttresses, and formed by accumulated debris. 



This and the surrounding mountains are composed of sandstone 

 and conglomerate resting on gneiss or granite, with intrusive dykes 

 of greenstone &c. When we consider that not a fossil of any kind 

 has been discovered or limestone in any form, and also the fact 

 already ascertained of the extension of this formation from Venezuela 

 to Brazil, and perhaps to Patagonia, it becomes an object of great 

 interest. 



Sehomburgk speaks of the sandstone range of Humerida to the 

 south of Roraima and of the sandstone of which Fort Sao Joaquim 

 is built on the Eio Branco, 2° south. Travellers on the Amazons 

 speak of fiat sandstone mountains as called by the natives " Guiana 

 Mountains ; " and from Darwin's observations on the more recent 

 formations of the valley of St. Crux, in Patagonia, I infer that 

 Eoraima represents deposits from the Cordilleras at an earlier 

 date and under similar conditions, and since it was uplifted it has 

 been eroded by the waters of the valleys of the Essequebo and 

 Orinoco. 



Eeturning to the extensive granite and gneiss formations, there is 

 on the Rewa, a tributary of the Rupununi, a ridge of granite which 

 presents several conical or pja'amidal hills, one of which, Ataraipu 

 or Devil's Rock, is nearly bare of vegetation, and rises about 900 feet 

 above the river and 550 feet above the forest by which it is sur- 

 rounded ; it is south-east of Roraima, and forms a part of the 

 Cunuku range. 



Discussion. 



Prof. Ramsay remarked upon the barreniiess, from a geological 

 point of view, of the district investigated by Mr. Sawkins, and 

 especially called attention to the absence of fossils in the stratified 

 rocks. He referred briefly to Mr. Sawkins 's labours in Trinidad and 

 Jamaica, and to his discovery of metamorphosed Miocene rocks in 

 the latter colony exactly analogous to the metamorpliic Eocene 

 rocks of the Alps. He was glad to see that the author had brought 

 forward examples of cross-bedding in metamorphic rocks, and con- 

 sidered that the results adduced were favourable to those views of 

 the metamorphic origin of granite which he had himself so long 

 upheld. 



Mr. D. FoKBES, on the contrary, considered that the facts brought 

 forward by Mr. Sawkins were confirmatory of the eruptive nature 

 of the granites observed. He added that a structure analogous to 

 the so-called cross-bedding was common in igneous rocks and even 

 in lavas. 



Mr. Tate remarked that in the country to the north of the dis- 

 trict described in the paper metamorphic rocks abound. He con- 

 sidered that the series of metamorphosed Jurassic rocks extends 

 across the whole north of South America, and perhaps into Cali- 

 fornia. Similar sandstones to those described occur in the basin of 

 the Orinoco, and contain fossils which show them to be of Miocene 



VOL. XXVII. — PART I. ^ H 



