22a 
THE OEOLOGIST. 
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 during 
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. 
MayW. — 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, 
Ehinoceros, Horse, Ox, and l)ecr, and a single phalanx of a small carni- 
vore, but no flint implements were discovered. 
2. " On a Deposit at Stroud containing Tlint Implements, Land and 
Freshwater Shells," etc.* By Mr. E. AVitchell, 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 l)y Ux. Joha Joues, of Gloucester. See ' Gcoloorist,' Vol. YI. 
p. 307.— Ed. Geol. 
