﻿Geological Society of London. 91 



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 tbe 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 cremdata, 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 horizon 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 maintained 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 simultaneously by 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 irregular 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. 



IL— January 10th, 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 Roj'al 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, 



