22G 



THE GEOLOGIST. 



Casts ; Calcareo-siliceous ; Calcareo-siliceous and Destructive ; Calcareo- 

 siliceous Casts. 



In describing these forms especial reference was made to those in which 

 the structures were more or less destroyed during the replacement (by- 

 silica) of the carbonate of lime which filled the interspaces, and duriDg 

 that of the ordinary hard parts of the coral. 



In explaining the nature and mode of formation of the large casts of 

 calices from Antigua, the author drew attention to the fact that the silifi- 

 cation is more intense on the surface and in the centre of the corallum 

 than in the intermediate region ; and, when examined microscopically, it 

 could be seen that the replacement of the carbonate of lime began by the 

 silica appearing as minute points in the centre of the interspaces and of 

 the sclerenchyma, and not on their surface. In conclusion, the relation of 

 hydrated silica to destructive forms of fossilization was discussed, toge- 

 ther with the influence of all the forms enumerated above in the preserva- 

 tion of organisms, and as one cause of the incompleteness of the geological 

 record. 



May 11. — 1. " On a Section with Mammalian Eemains near Thame." 

 By Mr. T. Codrington, F.G.S. — A railway-cutting through a hill between 

 Oxford and Thame having exposed a section of certain gravel- beds, from 

 which many mammalian remains were collected, the author now gave a 

 short description of the section, and a list of the bones he had obtained 

 from it. The hill is nearly surrounded by the Thame and two small tri- 

 butaries, and consists of Kimmeridge clay capped by a bed of coarse 

 gravel overlaid by sandy clay. The gravel consists of chalk-flints, pebbles 

 derived from the Lower Greensand, and fragments of mica-schist, etc., 

 indicating a northern-drift origin ; it contained many bones of Elephant, 

 Rhinoceros, Horse, Ox, and Deer, and a single phalanx of a small carni- 

 vore, but no flint implements were discovered. 



2. " On a Deposit at Stroud containing Flint Implements, Land and 

 Freshwater Shells," etc.* By Mr. E. Witchell, F.G-.S.— In the construc- 

 tion of a reservoir near the summit of the hill above the town of Stroud, the 

 author observed, about two feet from the surface, a deposit of tufa con- 

 taining land- shells, with a few freshwater bivalves ; in it he subsequently 

 discovered several flint flakes of a primitive type, and in the overlying 

 earth a few pieces of rude pottery. As the deposit is situated on the spur 

 of a hill nearly separated from the surrounding country by deep valleys, 

 and as Mr. Witchell considered it to be comparatively recent, he concluded 

 that it had been formed in a pond or lake, which had been caused by a 

 landslip from the higher ground producing a dam that stopped the down- 

 flow into the valley of the water of the neighbouring springs. 



3. " On the White Limestone of Jamaica, and its associated intrusive 

 rocks." By Mr. A. Lennox, F.G.S. — The White Limestone of Jamaica 

 was described as including a basement series of sandstones and shales, a 

 hard white limestone, a yellowish limestone, and an uppermost member 

 consisting of dark-red marl ; it was estimated to be at least 2500 feet thick ; 

 and the author stated that, at the junction of the calcareous rocks with the 

 granite, the former was often more or less altered, and this appeared to be 

 the best proof of the Tertiary age of the latter. Mr. Lennox then ad- 

 verted to a diagram-section of the rock-formations of Jamaica by the late 

 Mr. Barrett (Quart. Journ. Geol. Soc. vol. xix. p. 515), which he con- 



* Described already by Mr. John Jones, of Gloucester. See 'Geologist,' Vol. VI. 

 p. 307.— En. Geol. 



