284 



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- 

 wacke 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 of transition limestone imbedded in the upper portion of these 

 older formations : and from the phenomena described, certain facts 

 (important in the physical history of the mountain groups) become 

 securely established. 



1 . Great cracks were formed at a very ancient epoch, and probably 

 during the first period of elevation, diverging from the central regions 

 of the Lake Mountains; and such enormous shifts took place in the 

 position of the shattered strata, that in several instances the broken 

 ends of the same bed are more than a mile apart, the distance being 

 measured in a direction at right angles to the lines of bearing. In 

 after periods many of the existing valleys were scooped out upon the 

 lines of fracture. 



2; The central schistose groups abut in succession against the car- 

 boniferous zone ; and from this fact alone (independently of many 

 others bearing upon the same point), the two systems are proved to 

 be unconformable. 



3. The mean bearing of the great central groups, notwithstanding 

 their enormous dislocations, is, with very slight deviations, north-east 

 by east, and south-west by west. Now this is nearly the mean bear- 

 ing of the slate rocks of Cornwall, of the principal greywacke chains 

 of Wales and of the Isle of Man, and also of the entire greywacke' 



