﻿586 



DE. T. GROOM AND MR. P. LAKE ON THE 



[Nov, 1908, 



suddenly reduced to a few feet. Even if we should be mistaken 

 in identifying this crag as a part of the Craig-y-Pandy Ash, the 

 difficulty remains. Although the newest beds lie on the eastern 



side of ihe fault, it is impos- 

 isible, as the accompanying section 

 (fig. 16) will show, to explain 

 the relation of the beds upon the 

 hypothesis that there has been 

 any downward movement on its 

 eastern side. The figure shows the 

 Glyn Grit, the Dolhir thrust and 

 other faults, the Craig - y-Pandy 

 Ash and the Cwm-Clwyd Ash as 

 they appear upon the two faces of 

 the fault, the beds and faults upon 

 the eastern face being indicated 

 by thick lines and heavy shading, 

 and the beds and faults upon the 

 western face by thin lines and 

 light shading. It will be observed 

 that, on the western face, the 

 Dolhir thrust (tt) and the top of 

 the Glyn Grit (gg), if produced, 

 meet at the point 0. A vertically 

 downward displacement of about 

 ]380 feet on the east side would 

 bring the Dolhir thrust into the 

 position T, which it actually oc- 

 cupies ; but it would at the same 

 time bring the Glyn Grit nearly 

 to the same point, whereas it really 

 crops out at G, about 700 yards 

 to the north. Jio simple vertical 

 displacement can bring the lines 

 tt and gg at the same time into 

 the positions TT and GG re- 

 spectively ; and, since the distance 

 from ^ to G^ is greater than that 

 from t to g, it is clear that what- 

 ever vertical component the move- 

 ment may have had must have 

 been upwards on the eastern 

 side of the fault. 

 In order to determine the actual direction of the movement, all 

 that is necessary is to produce the lines GG and TT to meet in a 

 point corresponding to the point on the western side of the 

 fault. A displacement of the beds parallel to themselves that 

 would bring the point to the position will account for all the 

 facts observed. The point lies too far away to be shown upon 

 the figure ; but, considering the beds on the west of the fault to be 



