part 3] lower paleozoic oe arthog-dolgellev. 313 



The Ceunant Fault is probably a continuation of the one that 

 determines the long straight valley of the Wnion above Dolgelley 

 (fig. 7, p. 310), and which itself is a branch of the great Bala 

 Fault. The Wnion Valley leaves the fault-line near Dolgelley : 

 the line is continued, however, by the gorge of the Lower Ceunant, 

 and then, after an interval in which there is merely a hollow, but 

 not an actual gorge, it is further continued by the line of the 

 deeply-cut Gwernan Valley. 



At Llyn Grwernan the Ceunant and the Dolgelley Faults 

 converge, and ultimately coalesce under the alluvial deposits. The 

 result is that the two faults, throwing in opposite directions, 

 almost neutralize one another. The Ceunant Fault is the stronger 

 of the two, and accordingly the total effect is that of a slight 

 repetition, the fault-line being continued for another 5 miles along 

 a straight and deep valley, which eventually ends westwards as the 

 fault gradually dies away. 



The Ceunant Fault is clearly later than the intrusions, as regards 

 both its eastern and its western portions. Both sedimentary and 

 intrusive rocks are involved in a crush-breccia along the course of 

 the Lower Ceunant. Farther west the Crogenen granophyre is 

 repeated by the fault : the apparent vertical throw, determined 

 from relative positions of the upper boundary of the sill on 

 Mynydd Caer-go and T} r rau Mawr, being here about 1500 feet. 



The manner in which the Dolgelley and Ceunant Faults coalesce 

 and neutralize each other suggests that both were active at the 

 same time, which must have been later than the intrusions, the 

 result being to let down a wedge of country between the two 

 faults. But, judging from the manner in which the Dolgelley 

 Fault keeps parallel with the Derwas Fault, which was in part 

 clearly earlier than the intrusion, it seems reasonable to assume 

 that the Dolgelley Fault was itself active at more than one period. 



It is highly significant that all four faults — the Derwas, 

 Dolgelley, Ceunant, and IMynydd^-Grader North Faults — enter the 

 district on the east as strongly-marked features and with large 

 throws, and that they gradually diminish in power westwards, 

 finally disappearing within the limits of the area mapped. The 

 longitude where these faults disappear coincides with that at which 

 the strike changes from an approximately east-and-west direction 

 to one more nearly north-east and south-west. 



It is further noticeable that, just as the faults diminish west- 

 wards, so likewise do the intrusions, of which only one or two 

 examples extend more than a mile beyond the longitude at which 

 the faults die out. Moreover, in regard to the extrusive rocks, it 

 has already been shown that the thickness of the Lower Acid 

 Series decreases considerably to the west, and the same statement 

 seems to apply to the higher volcanic series. 



The various phenomena, namely : fi) change of strike, (ii) 

 disappearance of faults, (iii) disappearance of intrusions, and 



