344 THE GEOLOGIC A.L STKTJCTT7KE OP CENTEAL WALES. [Sept. I912, 



synclinal axis increases in importance in that direction. These 

 structures have exercised an important influence upon the topo- 

 graphy, the nature of which is not fully understood. 



EXPLANATION OF PLATE XXXIY. 



Greological map of Central Wales and the adjoining regions, on the 

 scale of 10 miles to the inch, or 1 : 633,600. 



Discussion. 



Dr. H. Lapworth welcomed the paper as an exceedingly valuable 

 contribution to the geology of Central Wales. The whole history 

 of geological research in this region furnished an illuminating 

 example of the way in which facts may stare one in the face, and 

 yet be unnoticed. The clue to the greater part of the Central 

 Wales area was the simplest that could be found, and was the fact 

 that, as shown by the work of Mrs. Shakespear, the Tarannon 

 formation of Central Wales was akin to the Gala Group of Southern 

 Scotland — not only in physical characters, but in attaining a thick- 

 ness of several thousands of feet. The original separation and 

 mapping in this district of the Tarannon Shales (by Ramsay and 

 Aveline) as a comparatively thin deposit lying between the 

 Llandovery and the Wenlock Series had, no doubt, led subsequently 

 to much confusion of ideas. 



The Author had stated that the Bala age of the rocks under- 

 lying the Gwastaden Grits at Rhayader had not been proved by 

 palaeontological evidence : as a matter of fact, the speaker quoted 

 in his paper DiplograjDtus foliaceus from beds south of Rhayader, 

 in material similar to that immediately below the Gwastaden 

 Series. 



This paper now provided us with a really satisfactory insight 

 into the geological structure of Central Wales ; and it was pleasant 

 to find that, although the Caban rocks Mere of local occurrence, 

 the main lithological types of the Bala, Llandovery, and Tarannon 

 rocks, as separated by the speaker in the Rhayader district, held 

 good over the greater part of the region. 



He hoped that the paper would not give an impression that the 

 geology of this area was now a closed book, for nine-tenths of the 

 1800 square miles had yet to be worked out in detail. 



Mr. E. A. Martin remarked on the excellent work which had 

 been done in the district by women-geologists, and desired to 

 emphasize our indebtedness to these earlier investigators. 



The Author, in reply, thanked Dr. Lapworth for his remarks, 

 but stated that, so far from his paper acting as a deterrent to other 

 geologists who wished to work in Central Wales, he thought that 

 the reverse would be the case, for he was fully cognizant of the 

 many points which still required urgent attention. 



