part 3] LOWER PALAEOZOIC OF ARTHOG-DOLGELLEY. 311 



found that in any case the diabase was not shifted to the same 

 extent as the sedimentary rocks. In part, therefore, the fault was 

 anterior to the intrusion. That movement was renewed later along 

 the same line, but subsequently to intrusion, is suggested by the 

 fact that there is a marked valley, or gorge, along the line of fault 

 where it passes across the igneous mass. 



As we trace the fault westwards it gradually transgresses into 

 higher strata, and then disappears beneath a prolongation of the 

 intrusion at Maes Angharad. Here, however, the intrusion is quite 

 small, and cannot conceal any great thickness of sedimentary rocks. 

 But, since the rocks on the north side of the intrusion belong to 

 the Dolgelley Beds and those on the south side to the Arenig 

 Basement Series, there is obviously extensive faulting underneath 

 the intrusion, the whole of the Tremadoc Slates being cut out, with 

 the result that some 900 feet of strata are missing. 



Still farther westwards, across the Tyn-y-llwyn depression, 

 Tremadoc Slates are in contact with the Lower Acid Series, the 

 Basement Beds being now cut out in their turn. Again the down- 

 throw is to the south, but again no fault is actually to be seen, as 

 it is covered by the granophyre- intrusion. The lower margin of 

 the granophyre is here almost coincident with the former line of 

 fault, wherefore a passing inspection of the map would suggest that 

 the granophyre is actually bounded by a fault. This is, however, 

 not the case, and the granophyre is not in the least affected by the 

 fault, as is clearly proved by the markedly metamorphosed con- 

 dition of the adjacent Tremadoc Slates. 



It is true that a little south of the line of older faulting there is 

 a zone in the granophyre marked by abundant quartz-veins, which 

 probably originated during some later movement along a line 

 parallel and close to the original line of weakness. Thus the 

 relation of the Derwas Fault to the granophyre falls into line with 

 the relation of the Derwas Fault to the diabase. 



Yet farther west, towards Waen Fechan, the fault emerges 

 from beneath the granophyre, and its effect is to cut out the 

 Basement Series all the way to Cae-Einion beyond the Grwynant 

 Valley. Along this stretch, however, the fault twice disappears 

 beneath an intrusion — the Cae-Einion Gelli-llwyd sill — which is 

 not itself affected by the fault on either occasion. The western- 

 most disappearance of the fault at Cae-Einion is final ; it evidently 

 dies away in a westerly direction. Altogether, then, the fault 

 disappears at least four times beneath different intrusive masses, 

 so that there can be no doubt as to the earlier age of the fault 

 relative to the intrusions. 



In the western portion of its course, where it is so frequently 

 concealed beneath intrusive rocks, the Derwas Fault naturally 

 makes no distinctive physical feature ; but farther east its line lies 

 along a marked hollow which descends past Gfyn-Malden, and 

 opens towards the silted-up head of the Mawddach Estuary at the 

 junction of the Wnion and Mawddach Valleys (fig. 7). Now, from 

 any of the numerous favourable view-points the opening of the 



