322 PROF. A. IT. COX AND ME. A. K. WELLS ON THE [vol. lxxvi,. 



'Fig. 4. Granophyre [C 228]. Near Craig-y-Castell. The rock is intermediate 

 between the markfieldite and eurite types. Idiomorphic albites and 

 granular quartz surrounded by fine-grained micropegmatite ; above 

 a xenolith, fine-grained and more basic, consisting of felspar, chlorite, 

 and magnetite, all granular. Crossed nicols. x 20. (See p. 297.) 



5. Felspathic modification, Tyn-y-llwyn type [C 226]. Smaller Tyn-y- 



llwyn intrusion. The ground-mass consists mainly of small felspars, 

 some idiomorphic, some granular, with rather abundant chlorite. 

 Also groups of larger irregular crystals of a curiously-twinned felspar 

 comparable with chequer-albite. In the illustration these larger 

 crystals almost fill the field. Crossed nicols. X 20. (See p. 299.) 



6. Chilled marginal rock [C 227]. Top of the larger Tyn-y-U\vyn intrusion. 



Felspar-phenocryst in a. matrix composed mainly of acicular felspar 

 and interstitial chlorite. Above is still finer-grained material, more 

 granular in texture. Crossed nicols. X 20. (See p. 301.) 



Plate XX. 



Geological nap of the Arthog-Dolgelley district, on the scale of 3 inches 

 to the mile, or 1 : 21,120. 



Discussion. 



Prof. W. W. Watts desired to congratulate the Authors on 

 both the subject-matter of the paper and the manner of its presen- 

 tation to the Society in the time at their disposal. He thought 

 that they had been wise in clearing up the structure of the foothills 

 before dealing with the main range, although the saturation of the 

 rocks with intrusive material rendered it by no means eas} r to 

 unravel the succession and structure there. 



It was perhaps to be expected that the Tremadoc succession 

 should fit in with that at Arenig and Portmadoc ; but the ap- 

 pearance of a new Dictyonema Band was of interest, and he asked 

 whether the species agreed with those of the Lower Band. The 

 Authors appeared to have given satisfactory evidence that there 

 was no Tremadoc vulcanicity ; but he wished to hear the reasons 

 which had led to the conclusion that the main period of intrusion 

 was pre-Silurian, as such a conclusion was not in accordance with 

 his own experience. The series of intrusive rocks ranging from 

 basic to acid worked out by the Authors was of interest in con- 

 nexion with the stratification of the intrusions — a phenomenon 

 noticed by Dr. A. Harker in the Lleyn and by the speaker in Shrop- 

 shire. The resemblance of some slides shoAvn to the ' sj^enite- 1 

 group in Charnwood Forest was remarkable and illuminating, as the 

 rocks in that area also had a wide range in composition. Finally, 

 the speaker called attention to the general form of the intrusive 

 masses as shown in the sections on the screen. 



Mr. J. F. N. Green said that, as he had described a Bala un- 

 conformity in the Lake District and had there referred a number 

 of intrusions, previously said to be Devonian or Tertiary, to a pre- 

 Upper Bala age, he was naturally interested to learn that, in the 



