91 
Geological Society of London. 
places a kind of double cleavage affecting the lower series, but not 
the upper, and also fragments of cleaved mudstone included in the 
upper, from which he inferred a disturbance of the older rocks 
previous to the deposition of the newer. He exhibited a selection of 
fossils, and said that immediately below the Corwen beds there were 
none but Bala fossils. In the Corwen beds all the few fossils found 
were common to the Llandovery rocks, some of them, as Meristella 
crassa and Petraia crenulata , being peculiar to that formation. In 
the flaggy slates above the Pale Slates he had found Graptolites and 
Orthoceratites of the same species as those found in the Denbigh 
Flags. He considered that the Corwen Beds were on the liorizon of 
the May Hill or Llandovery group, and should be taken as the 
base of the Silurian, thus including in the Pale Slates or Tarannon 
Shale a thick series which intervened between the Corwen Beds and 
the flaggy slates of Penyglog. 
4. “On Mineral Veins.” By W. Morgan, Esq. Communicated 
by Warington W. Smyth, Esq., F.R.S., F.G.S. 
The author maintaiued that no one theory can be accepted in 
explanation of the formation of mineral veins ; and that whilst in 
some cases their formation may be due to the presence of pre-existent 
fissures induced by shifting of the containing rock, in others any 
such explanation is insufficient, as he thought the means by which 
the sides of such fissures were kept apart could not be easily indicated. 
The point upon which he especially insisted in connexion with this 
question was the presence of “ horses ” in many mineral veins. He 
advocated the view that the walls of veins were in close proximity 
in their earliest stage, and that the enlargement and infilling of the 
veins took place simultaneouslyby the Segregation of materials derived 
from the adjacent rock, supplemented, perhaps, by a tension or 
tendency to Separation caused by slow contraction of the latter. 
Instead of a fissure he assumed the presence of an irregulär surface 
of least resistance or of electrical action, at which the vein matter 
might collect at first as a mere film. In this way, he thought, the 
vein might increase and its walls might recede simply by the aggre- 
gation of the vein-matter itself, and in general in proportion to the 
degree of mineral Saturation of the adjacent rocks. 
n. — January lOth, 1877. — Prof. P. Martin Duncan, M.B., F.R.S., 
President, in the Chair. — The following Communications were read : — 
1. “ On Gigantic Land-Tortoises and a small Freshwater Species 
from the Ossiferous Caverns of Malta, together with a List of the Fossil 
Fauna, and a Note on Chelonian Remains from the Rock-cavities of 
Gibraltar.” By A. Leith Adams, Esq., M.B., F.R.S., F.G.S., Pro- 
fessor of Zoology in the Royal College of Science, Dublin. 
The author described three extinct species of Tortoises from the 
Maltese rock-cavities, one of which was of gigantic proportions, and 
equalled in size any of the living or extinct land Chelonians from 
the Indian or Pacific islands. The characteristic peculiarity in the 
two larger species is a greater robustness of the long bones as com- 
pared with the denizens of the Mascarene and Galapagos islands, 
