382 MESSES. A. V. JENNINGS AND G. J. WILLIAMS 



seen evidence of the occurrence of cleaved Cambrian rocks in the 

 agglomerates. He criticized the view that the Portmadoc Grit 

 could be traced as shown on the new Survey-Map at the base of the 

 Arenig all along the Ffestiniog district, and explained that east of 

 Garth it was split up by the intercalation of shale ; that its represen- 

 tative consisted of less quartz and more felspathic material near 

 Tanybwlch ; while farther east it was difficult to identify it at all. 



Dr. HiCKs congratulated the Authors on the important results 

 obtained by them in their investigations in the Pfestiniog district. 

 In addition to correcting some inaccuracies on the Survey-Maps, 

 and defining the exact position of beds hitherto doubtfully placed, 

 they had brought forward conclusive evidence to show that the 

 granite (so-called "syenite") of Tan-y-grisiau was intrusive in the 

 surrounding rocks. This mass, in a paper in 1879, he had placed 

 with the pre-Cambrian rocks, mainly on the strength of evidence 

 supplied to him by the late Mr. Tawney ; but in the map which he 

 prepared for the excursion of the International Geological Congress 

 in 1888, he (Dr. Hicks) corrected that error and indicated it as 

 an intrusive mass as properly shown on the Survey-Maps. 

 As the Cambrian grits in the neighbouring Harlech Moun- 

 tains are largely made up of broken quartz and felspar, and of 

 pebbles of granite, it seems probable that this intrusive granite mass 

 at Tan-y-grisiau. may after all be but a softened or fused portion of 

 a pre-Cambrian floor (composed of granitic and gneissic rocks), which 

 was intruded into the overlying rocks mainly as the result of pres- 

 sure during subsequent earth-movements. The similarity between 

 some of the fragments in the Harlech conglomerates and portions of 

 this granitic mass is very striking. 



Mr. Sherborn said that, with regard to Dr. Hicks's statement as 

 to his omission of the pre-Cambrian area in the map printed in the 

 Excursion Heport of the last Geological Congress, he must, in 

 justice to Mr. Jennings, remind the Society that the printed matter 

 referred to was marked " Epreuve sujette a revision " and could in 

 no way be quoted until properly published in the Report of that 

 Congress. 



The President remarked that the present paper formed another 

 good illustration of the kind of patient detailed work that was 

 required in extension and correction of the broad outlines traced by 

 the Geological Survey. He spoke in warm terms of the Survey-Maps 

 of North Wales, but admitted that they could now be much improved. 

 Alluding to one or two parts of the paper about which there might 

 be differences of opinion, he pointed out that in discussing the nature 

 and amount of alteration in the ring of contact-metamorphism, we 

 must bear in mind that, besides the capacity of the surrounding rocks 

 for being metamorphosed, there were probably also initial differences 

 in the capacity of the eruptive rocks to produce alteration. 



Mr. Williams thanked the Fellows of the Society, on Mr. Jennings' 

 and his own behalf, for the manner in which the paper had been 

 received. 



In reply to Prof. Hughes he said that the cleaved fragments of 



