Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/583

 feet from the summit to the inclined 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.

Schomburgk 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 Rio Branco, 2° south. Travellers on the Amazons speak of flat 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 Roraima 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.

Returning 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 pyramidal 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 surrounded ; it is south-east of Roraima, and forms a part of the Cunuku range.

Discussion.

Prof. Ramsay remarked upon the barrenness, 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 metamorphic Eocene rocks of the Alps. He was glad to see that the author had brought forward examples of cross-bedding in metamorphic rocks, and considered that the results adduced were favourable to those views of the metamorphic origin of granite which he had himself so long- upheld.

Mr. D. Forbes, 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 district described in the paper metamorphic rocks abound. He considered that the series of metamorphosed Jurassic rocks extends across the whole north of South America, and perhaps into California. Similar sandstones to those described occur in the basin of the Orinoco, and contain fossils which show them to be of Miocene

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