320 PKOF. A. H. COX AXD ME. A. K. WELLS OS THE [vol. lxxvi, 



The earlier intrusions were diabases of normal Welsh type. 

 These were followed b} r a set of intrusions derived from a grano- 

 pliyrie magma which was undergoing progressive and deep-seated 

 differentiation. Various basic and intermediate differentiates were 

 drawn off, and intruded in order as a set of minor intrusions. The 

 minor intrusions together contain all transitions, from thoroughly 

 basic rocks with little or no quartz, through intermediate rocks — 

 markfieldites — to thoroughly acid rocks. In general, the rocks in 

 •each sill are more basic than those in the sills above ; further, there 

 is a stratiform variation within the limits of individual intrusions, 

 the more basic rocks being alwa3 7 s at the bottom. These minor 

 intrusions were followed by the main mass of acid material — the 

 Oogenen granophyre — which forms a laccolite over 1200 feet thick. 



Structural and petrographical considerations point to a pre- 

 Upper Bala age for all the intrusive rocks. 



The area is dissected by faults of three main types, the most 

 important being those that are almost parallel to the strike. 

 Dip-faults and north-west faults are of subordinate importance. 



The process of tilting, folding, and faulting commenced during 

 the Ordovician Period. These movements were followed by the 

 intrusion of the hypabyssal rocks. The main folding, the cleavage, 

 and the main faulting were posterior to the intrusion of the igneous 

 rocks, but followed along the same lines as the earlier movements. 



In conclusion, we wish to express our indebtedness to Prof. S. H. 

 Reynolds for the loan of his 6-inch geological maps of the country 

 near Dolgelley ; the lines on his maps l have been adopted, with 

 but slight modification, in the preparation of PI. XX. We are 

 indebted to Mr. Prank Raw and to Dr. V. C. Illing for their 

 Identifications of certain of the Tremadoc trilobites ; to Mr. 

 John Pringle for examining many of the graptolites ; and to 

 Mr. 1ST Or. Blackwell for photographs. We wish also to thank 

 Mr. and Mrs. Griffith Pugh, Penrhyn-gwyn, for their hospitality 

 during our visits to the district. One of us (A. H. C) wishes to 

 .acknowledge his indebtedness for grants from the Royal Society, 

 and from the Dixon Fund of the Universit}^ of London, which 

 helped to defray a part of the expenses during the earlier stages of 

 the investigation. 



EXPLANATION OF PLATES XVI-XX. 



Plate XVI. 

 View looking eastwards across the Gwynant Valley to Graig-Llwyd. 

 'The] sky-line corresponds to part of the line of section, CD, fig. 1, p. 258. 

 a. Ffestiniog Beds. bb. The Craig-y-Castell markfieldite intrusion 

 covering Dolgelley Beds, but not extending down into the valley, 

 c. Graig-Llwyd, the Niobe-Heds scarp. d. Asaphellus-Beds scarp, 

 e. Scarp of Waen-Fechan intrusion. (See pp. 255, 262, 289.) 



1 P. Lake & S. H. Eeynolds, Q. J. G. S. vol. Hi (1896) pi. xxiv ; and ibid. 

 vol. lxviii (1912) pi. xxxv. 



