CHAP. XIII 
BALA ERUPTIONS OF CAERNARVONSHIRE 
213 
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 w’eathered 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 bauds among the true tuffs may be seen on the eastern 
precipices of Snowdon above Glas-lyii. 
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 Lleyu, another group of 
volcanic rocks exists which may have been in a general sense contem- 
poraneous with those of the Snowdon region, but which were certainly 
erupted from independent vents. Mr. Harker lias described them as quartz- 
less pyroxene - andesites, sometimes markedly cellular, and though their 
geological relations are rather obscure, he regards them as lava-flows inter- 
bedded among strata of Bala age and occurring below the chief rhyolites of 
the district. If this he their true position, they indicate the outflow of 
much less highly siliceous lavas before the eruption of the acid felsites. In 
the Snowdon area any such intermediate, rocks which may have been 
poured out before the time of the felsitic outflows have been buried under 
these. 
The tuffs of the Bala series in Caernarvonshire have not received the 
same attention as the lavas. One of the first results of a more careful 
study of them will probably he a modification of the published maps by a 
reduction of the area over which these rocks have been represented. They 
range from coarse volcanic breccias to exceedingly fine compacted volcanic 
dust, which cannot easily be distinguished, either in the field or under the 
microscope, from the finer crushed forms of felsite. Among the oldest tuffs 
pieces of dark blue shale as well as of felsite may be recognized, pointing to 
the explosions by which the vents were drilled through the older Silurian 
sediments already deposited and consolidated. Sometimes, indeed, they 
recall the dark slate-tuffs of Cader Idris, like which they are plentifully 
sprinkled with kaolinized felspar crystals. The beds of volcanic breccia 
intercalated between the lower felsites of Snowdon include magnificent 
examples of the accumulation of coarse volcanic detritus. The blocks of 
various felsites in them are often a yard or more in diameter. Among the 
