part 2] PALAEOZOIC bocks of the lla^v^gollen district. 223 



great changes in its level, and by lateral movement, great changes 

 in the relative position of the blocks of country into which it vs^as 

 already divided. These changes are to some extent self-evident 

 from the map. Without going into the detailed structure of the 

 Carboniferous, we can make the following statements : — 



(1) All the master-faults (see PI. V) moved again, aitd the 



fault-lines were propagated through the Carboniferous 

 cover as major faults. 



(2) The adjustment-faults which adjoin the Carboniferous can 



be shown to have moved, and by inference, therefore, all 

 the adjustment-faults probably did so. 



(3) Many of the adjustment-faults are curved, both in the older 



rocks (see PL V) and in the Carboniferous, and their 

 peculiarities can be best explained b}^ the assumption that 

 they result from the torsion of the region. 



(4) The down-sinking of the Yale-of-Clwyd graben along the 



Vale-of-Clwyd and Llanelidan Faults shows how a major 

 synclinal tendency was expressed at this time by faulting 

 rather than by folding. 



EXPLANATION OF PLATES III-V. 



Plate III. 



Map of area round Moel-y-faen, illustrating the structure of the northern 

 limb of the Llangollen Synclinorium (see p. 214), on the approximate 

 scale of 1 inch to 6QQ yards, or 1 : 24,000. 



Plate IV. 



North- to- south serial sections across the Llangollen Synclinorium, on the 

 scale of 1 inch to the mile, or 1 : 63,360. (See pp. 212, 214.) 



Plate V. 



Geological sketch-map of the country around Llangollen, on the scale of 

 1 inch to the mile, or 1 : 63,360. [The distinctive colour-bars have been 

 accidentally omitted from the narrow strip of Cor^w^n Grit, south of 

 Bonwm.] 



DlSCUSSIOlS^. 



The Secretary read the following letter received from Mr. 

 Beei^aed Smith: — 



' It should be pointed out that Dr. Wills had actually commenced to map 

 the Ordovician and Valentian rocks of Cyrn-y- Brain, in his spare time, before 

 it was decided to map the ground officially. He was therefore forestalled by 

 the Geological Survey ; and it was my fortune to have allotted to me certain 

 areas that he had planned to examine himself. Hence the collaboration that 

 has given rise to this paper. The area covered by the latter, however, is 

 much greater than Dr. Wills had contemplated mapping by himself, and the 

 combined experience of two, if not three, surveyors — gained not only in this 

 area, but in a more extensive field upon three sides of it — has been utilized. 



' The structures now described may be taken as typical of what has 

 happened in the district : they are, however, but a portion of a more wide- 

 spread system in which a large part of North Wales is involved. It had 

 been hoped that another paper written by Mr. C. B. Wedd and myself, would 

 have been ready for communication to the Society at about this date, but that 



