212 
THE SILURIAN VOLCANOES 
BOOK IV 
While by far the larger porportion of the Caernarvonshire lavas con- 
sists of thoroughly acid rocks, the oldest outflows are much less acid than 
those erupted at the height of the volcanic activity, when the rocks of Snowdon 
were poured forth.^ 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 Bamsay has shown the existence of an upper “ felstone ” or “ fel- 
spathic porphyry,” almost entirely removed by denudation, but of which 
outliers occur on Crib-goch, Lliwedd, and other crests around Snowdon, and 
likewise on Moel Hebog.“ 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 an 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 TOO to 150 feet (6 in Big. 56). 
The apex of \r Wyddfa, the peak of Snowdon, consists of fossiliferous shale 
1. Grits and slates ; 2. Felsits with good flow-structure ; 8. Volcanic tuffs • 
felsite and andesite ; (5. Group of andesitic lavas on suminib of Snowdon 
4. Felsite; 5. Tuffs with sheets of 
; 7. Intrusive “ greenstones.” 
lying on a dull grey rock that weathers with elongated vesicles, somewhat 
like a cleaved amygdaloid, but a good deal decomposed. A thin slice of this 
latter rock shows under the microscope irregular grains and microlites of 
felspar, with a few grains of quartz, the whole much sheared and calcified 
Below this bed comes a felsite, or devitrified obsidian, showing in places 
good spherulitic structure, and followed by a grey amygdaloid. The latter 
IS a markedly cellular rock, and, though rather decayed, shows under the 
microscope a microlitic 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 at the top, but at many horizons throughout this supposed thick 
1 Mr. Harker, op. cit. p. 127. 
“ Mem. Geol. Surv. rol. iii. 2nd edit. pp. 141, 144, 145, 147, 161. 
f IT- He refers also to lavas occupying a similar position 
at Nant Gwynaiit and Moel Hebog ; but he adds that he had not had an opportunity of studying 
After the Geological Survey Section (Horizont. Sect. Sheet 28), slightly modified. 
