378 Reports and Proceedings, 



in the older rocks whole genera of Lamellibranchs are confined to horizons and 

 localities ■which are not cut off by stratigraphical breaks, such as would allow us to 

 think it at all probable that they can be characterized by peculiar genera. He 

 thought the scarceness and in-egular occurrence of Lamellibranchs in the older rocks 

 could be best explained on the supposition that those portions of the older deposits 

 which were least favourable to Lamellibranchs happened to be those now chiefly 

 exposed to our search, and that those few portions are only in part worked out. 



Mr. Jenkins observed that in thick deposits there was a far greater likelihood of 

 numerous forms being present than in thin, for thickness meant time, and time meant 

 variation. 



Prof. Morris dissented from this view, as in thin littoral deposits an enormous 

 number of shells might be present, while in beds formed in deep sea they might be 

 almost entirely absent. 



2. " Geological Observations on British Guiana." By James G. 

 Sawkins, Esq., F.G.S. 



In this paper the author gave a general account of his explora- 

 tions of the Geology of British Guiana when engaged in making the 

 Geological Survey of that colony. He described the rocks met with 

 during excursions in the Pomeroon district, along the course of the 

 Cuyuni and Mazuruni rivers, on the Demerara river, on the Essequibo 

 and its tributaries, on the Eupununi river, and among the southern 

 mountains. The rocks exposed consist of granites and metamorphic 

 rocks, overlain by a sandstone, which forms high mountains in the 

 middle part of the colony, and is regarded by the author as probably 

 identical, or nearly identical, with the sandstone stretching through 

 Venezuela and Brazil, and observed by Mr. Darwin in Patagonia. 



Discussion. — Prof. Eamsay 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 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 age. Mr. Tate did not consider 

 these sandstones as the equivalent of the Patagonian sandstones, as from the shells 

 contained in the latter they would appear to be Pliocene or Pleistocene. 



Mr. Sawkins, in reply to a question from Mr. Tate, stated that the only gold found 

 in the country had probably been carried down from the well-known gold district of 

 TJpata. He also entered into a few additional details connected with the chief points 

 in his paper, dwelling especially upon the physical features of the country, in 

 illustration of which several landscape drawings were exhibited. 



n.— June 7, 1871.— Joseph Prestwich, Esq., F.E.S., President, 

 in the Chair. The following communications were read : — 1. " On 

 the persistence of Caryophyllia cylindracea, Eeuss, a Cretaceous Coral, 

 in the Coral-fauna of the Deep Sea." By P. Martin Duncan, M.B. 

 Lond., F.E.S., F.G.S., Professor of Geology in King's College, 

 London. 



