"Vol. 6S.'] THE GEOLOGY OF MTNTDD-Y-GADER. 345 



19. The Geology of Mynydd-y-Gabee, Dolgelly. By Philip Lake, 

 M.A., F.G.S., and Sidney Hugh Eeynolds, M.A., P.G.S., 

 Professor of Geology in the University of Bristol. (Bead 

 May 1st, 1912.) 



[Plates XXXV-XXXIX.] 

 Contents. 



I. Introduction 345 



II. The Geology of Mynydd-y-Gader 345 



(1) The General Structure 345 



(2) The Stratified Eocks 347 



(3) The Forms and Relations of the Intrusive Masses. 348 



III. The Petrology of the Igneous Eocks 353 



(a) The Intrusive Eocks 353 



(b) The Contemporaneous Eocks 357 



IV. Comparison with other Areas 361 



I. Inteodtjction. 



In our previous paper ^ upon this district we described the geology 

 of the area which lies between Dolgelly and Mynydd-y-Gader, a 

 ridge which rises prominently about a mile and a half south of 

 the town, in front of the precipices of Cader Idris. But our obser- 

 vations ended at the foot of Mynydd-y-Gader, and the relations of 

 the dolerite which forms the mass of that ridge to the sedimentary 

 beds of the lower-lying ground were by no means clear. We were 

 led, therefore, to extend our map somewhat farther southwards, 

 and it is chiefly with this southern part of the district that we are 

 now concerned. 



Much of the field-work was done by the two authors in company, 

 but each has visited the district at other times alone. One of the 

 authors (P. L.) is primarily responsible for the interpretation of 

 the structure, the other (S. H. B.) for the petrological examination 

 of the rocks. 



Beferences to the work of other observers will be found in our 

 previous paper. 



II. The Geology op Mynydd-y-Gabek. (P. L.) 



(1) The General Structure. 



Dolgelly stands in the valley of the Wnion. The southern side 

 of the valley is the northern edge of a broad irregular plateau, 

 about 500 feet above sea-level. On the surface of this plateau 

 the Odyn and the Aran, with its tributary the Ceunant, have cut 

 broad and shallow valleys, which near the edge become deep and 



^ ' The Lirigula Flags & Igneous Eocks of the Neighbourhood of Dolgelly ' 

 Q. J. G. S. vol. lii (1896) pp. 511-21. 



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