﻿Vol. 64.] THE BALA AND LLANDOVEET EOCZS OF GLTN CEIEIOG. 575 



the drift and alluvium that cover the lower slopes of the hill ; but 

 a single block of limestone in the wood near Dol-y-wern appears 

 to be nearly, if not quite, in situ. 



Near Dol-y-wern, the Cae-mawr Pault crosses the valley and 

 throws all the beds to the south ; and the only place to the east 

 of this fault where we have discovered the grit actually in place is 

 Gwernydd-gymmal, where it is visible in the farmyard. Exposures 

 in this part of the district are very poor ; but the occurrence of 

 occasional outcrops of the Eron-Frys Slates enables us to insert the 

 line of the grit with some degree of^accuracy. 



The Glyn Grit is variable in thickness, and towards Want Llafar, 

 where it may attain a thickness of l50 or 160 feet, the rapidity of 

 the variation becomes remarkable. In the accompanying diagram 

 (fig.lO, p. 574) the dotted areas represent the outcrops of the grit 

 on Mynydd Fron Frys and on the slopes of ISFant Llafar, the two 

 areas being separated by the alluvium of the Ceiriog. By joining 

 the points where the upper edge of the grit crosses the contour- 

 lines of 1000 feet, 900 feet, etc., we obtain approximate contour- 

 lines of the upper surface of the grit, shown in the diagram as 

 thin continuous lines. Contour-lines of the base of the grit may 

 be obtained in a similar fashion, and these are shown in the 

 diagram as thin broken lines. 



It will be observed that the contonr-lines of the upper surface 

 are nearly straight and separated by approximately equal intervals, 

 indicating that this surface is almost a plane. On Mynydd Fron 

 Frys the contour-lines of the base are nearly parallel to those of the 

 top and all but equidistant, showing that here the thickness is 

 nearly uniform ; but towards Nant Llafar they diverge rapidly. 

 At any point where the contour-line 1000 of the upper surface 

 crosses the contour-line 900 of the lower surface, it is evident that 

 the top of the grit is 100 feet above the base. Assuming that the 

 dip is 16°, the actual thickness of the grit at such a point will be 

 approximately 96 feet. In this way are obtained the heavy broken 

 lines marked 96 and 48, along which the thickness of the grit is 

 96 feet and 48 feet respectively. 



An unconformity, either at the base or at the top of the grit, 

 would explain the variation. The fauna of the grit is similar to 

 that of the Dolhir Beds, and there appears to be a passage from 

 the one to the other. The fauna of the Fron- Frys Slates is very 

 different, and it is probable, therefore, that if there is any un- 

 conformity it is at the top of the grit ; but it is not visible in 

 any single section. In order, if possible, to determine the relations 

 of the Glyn Grit and the Fron-Frys Slates, we made an excavation 

 at the boundary in Nant Llafar, with the result shown in fig. 11 

 (p. 576). There is no visible sign of any interruption in deposition. 



About 500 yards west of the Glyn- Valley Hotel there is a 

 distinct break in the heart of the grit, and the beds above the break 

 have the appearance of resting unconformably on the beds below. 

 But we can detect no difference, either lithological or palaeonco- 

 logical, between the beds on the two sides of this apparent uncon- 



