ANNIVEKSARY ADDKES8 OF THE PRESIDENT. I 23 



lying oil a dull grey rock that weathers with elongated vesicles, 

 somewhat like a cleaved amygdaloid, but a good deal decom- 

 posed. A thin slice of this latter rock shows under the microscope 

 irregular grains and microliths of felspar, with a few grains of 

 quartz, the whole much sheared and calcified. Below this bed comes 

 a felsite, or devitriticd obsidian, showing in places good spherulitic 

 structure, and followed by a grey amygdaloid. The latter is 

 £L markedly cellular rock, and, though rather decayed, shows 

 under the microscope a microlithic felspathic groundmass, through 

 which granules of magnetite are dispersed. 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 

 Bt 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 them 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 tra- 

 chytes, 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. 



Far to the south-west, in the promontory of Lleyn, another group 

 of volcanic rocks exists which may have been in a general sense 

 contemporaneous with those of the Snowdon region, but which 

 were certainly erupted from independent vents. Mr. Harker 

 has described them as quartzless pyroxene-andesites, sometimes 

 markedly cellular, and though their geological relations are rather 



