122 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



Another remarkable aspect of the felsites is that nodular structure 

 so often to be seen among them, and regarding the origin of which 

 so much has already been written. I agree with Professor Cole and 

 Mr. Harker in looking upon the " nodules " as derived from original 

 spherulites by a process of alteration, of which almost every succes- 

 sive stage may be traced until the original substance of the rock has 

 been converted into a flinty or agate-like material. If this be the 

 true explanation of the structure, some of the original lavas must have 

 exhibited perlitic and spherulitic forms on a gigantic scale. There 

 can, I think, be little doubt that this peculiar structure was very gene- 

 rally misunderstood by the earlier observers, who naturally looked 

 upon it as of clastic origin, and who therefore believed that large 

 beds of rock consisted of volcanic conglomerate which we should now 

 map as nodular felsite (pyromeride) *. 



While by far the larger proportion of the Caernarvonshire lavas- 

 are thoroughly acid rocks, the oldest outflows are much less sa 

 than those erupted at the height of the volcanic activity when the 

 rocks of Snowdon were poured forth f. But towards the close of 

 the period there was apparently a falling off in the acidity of the 

 magma, for at the top of the group the andesitic lavas to which 

 I have already alluded are encountered. Sir Andrew E-amsay 

 has shown the existence of an upper " felstone " or " felspathic por- 

 phyry," almost entirely removed by denudation, but of which 

 an outlier occurs on Crib-goch, Lliwedd, and other crests around 

 Snowdon, and likewise on Moel Hebog J. Mr. Harker alludes to 

 these remnants, and speaks of them as less acid than the older lavas, 

 but he gives no details as to their structure and composition §. In 

 a recent examination of Snowdon I was surprised to find that the 

 summit of the mountain, instead of consisting of bedded ashes as 

 hitherto represented, is formed of a group of lava-sheets having a 

 total thickness of perhaps from 100 to 150 feet. The apex of 

 Yr "Wyddfa, the peak of Snowdon, consists of fossiliferous shale 



* Another source of error may probably be traced in the occasional brec- 

 ciated structure of the felsites, which has been mistaken lor true volcanic breccia, 

 but which can be traced disappearing into the solid rock. Sometimes this 

 structure has resulted from the breaking up of the lenticles of flow, sometime* 

 from later crushing. 



t Harker, op. cit. p. 127. 



+ Mem. Geol. Surv. vol. iii. 2nd edit. pp. 141, 144, 145, 147, 161. 



§ ' Bala Volcanic Series,' pp. 10, 23, 125. He refers also to lavas occupying a 

 similar position at Nant Gwynant and MoeLHebog ; but he adds that he had 

 not had an opportunity of studying them. 



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