284- Geological Society. 



an elvan or porphyry dyke near St. Austell). The regular metallife- 

 rous lodes were probably once but cracks and fissures produced du- 

 ring some periods of elevation j and how they have been filled up is 

 perhaps a question beyond our scrutiny. But after the important ex- 

 periments of Mr. Fox, there can, I think, be no doubt that the great 

 vertical dykes of metallic ore, which rake through so many portions 

 of the county, owe their existence, at least in part, to some grand de- 

 velopment of electro-chemical power. 



In all the crystalline granitoid rocks of Cornwall there are also 

 many masses and " veins of segregation." Such are the great contem- 

 poraneous masses and veins of schorl rock j and some of these are 

 . metalliferous. The decomposing granite of St. Austell Moor is tra- 

 versed, and sometimes entirely superseded, by innumerable veins of 

 this description. Upon these lines of schorl rock there is often aggre- 

 gated a certain quantity of oxide of tin, which sometimes diffuses itself 

 laterally into the substance of the contiguous granite. After examin- 

 ing this district with Professor Whewell during the summer of 1 828, 

 we left it in the conviction that several of the neighbouring tin works 

 were opened not upon true lodes, but upon "veins of segregation.'" I 

 only throw out these remarks as hints for future inquiry j as the sub- 

 jects introduced by the memoir of Mr. Weaver are of vast importance, 

 and have been unfortunately but seldom brought under the conside- 

 ration of this Society. 



A paper by Mr. Alfred Thomas gives us some new details con- 

 nected with the structure of the northern parts of Pembrokeshire. 

 His descriptions are illustrated by a geological map, and a section 

 extending north and south from Cardigan to St. Gowan's Head. By 

 help of this section we are conducted, in a descending order, from the 

 higher part of the coal series with subordinate beds of anthracite, 

 through the mountain limestone, the old red sandstone and conglo- 

 merates, and the transition limestone with Trilobites, down to grey- 

 wack6 and greywacke* slate. All these formations are occasionally 

 traversed by masses of trap producing contortions and changes of 

 structure among the rocks with which they are in contact. 



In a communication read very recently to the Society, I have en- 

 deavoured to explain the structure of the Lake Mountains and the 

 period of their first elevation the manner in which, during a sub- 

 sequent period of elevation, they were separated from the great 

 calcareous chain of the north and the relations they still bear to it 

 through the intervention of a carboniferous zone. In conformity with 

 the system first published by Mr. Otley of Keswick, I have shown 

 that the greater part of the central region of the Lake Mountains is 

 occupied by three distinct groups of stratified rocks of a slaty texture : 

 and I have further shown, that crystalline unstratified masses form the 

 true mineralogical centres of these great groups that by the protru- 

 sion of these masses the schistose formations have been elevated into 

 the positions they now occupy and that a true mineralogical axis 

 may be traced through the oldest division of the slate rocks, on each 

 side of which the several formations, as far as they are developed, are 

 arranged symmetrically. I have traced in great detail the range of a 



band 



