microlitic felspathic groundmass, through which granules of magnetite are dispersed. [Footnote 205: Mr. Harker, _op. cit._ p. 127.] [Footnote 206: _Mem. Geol. Surv._ vol. iii. 2nd edit. pp. 141, 144, 145, 147, 161.] [Footnote 207: _Bala Volcanic Series_, pp. 10, 23, 125. He refers also to lavas occupying a similar position at Nant Gwynant and Moel Hebog; but he adds that he had not had an opportunity of studying them.] [Illustration: Fig. 56.--Section of Snowdon.[208] 1. Grits and slates; 2. Felsite with good flow-structure; 3. Volcanic tuffs; 4. Felsite; 5. Tuffs with sheets of felsite and andesite; 6. Group of andesitic lavas on summit of Snowdon; 7. Intrusive "greenstones."] [Footnote 208: After the Geological Survey Section (Horizont. Sect. Sheet 28), slightly modified.] Underneath this upper group of lavas lie the tuffs for which Snowdon has been so long celebrated. But, as I have already stated, there does not appear to me to be such a continuous thickness of fragmental material as has been supposed. There cannot, I think, be any doubt that not only at the top, but at many horizons throughout this supposed thick accumulation of tuff, some of the beds of rock are really lava-flows. Some of these lavas have suffered considerably from the cleavage which has affected the whole of the rocks of the mountain, while the results of centuries of atmospheric disintegration, so active in that high exposed locality, have still further contributed to alter them. They consequently present on their weathered faces a resemblance to the pyroclastic rocks among which they lie. Where, however, the lavas are thicker and more massive, and have resisted cleavage better, some of them appear as cellular dull grey andesites or trachytes, while a few are felsites. Many instructive sections of such bands among the true tuffs may be seen on the eastern precipices of Snowdon above Glas-lyn. It thus appears that the latest lavas which flowed from the Snowdonian vent were, on the whole, decidedly more basic than the main body of felsites that immediately preceded them. They occur also in thinner sheets, and are far more abundantly accompanied with ashes. At the same time it is deserving of special notice that among these less acid outflows there are intercalated sheets of felsite, and that some of these still retain the spherulitic structure formed by the devitrification of an original volcanic glass.