Coder Sen wyn. 
CHAP, xm 
BALA ERUPTIONS OF THE BERWYN HILLS 
219 
thickness of 300 feet and separated from the next band by sometimes 1000 
or 1500 feet of non- volcanic sediment. 
These lower lavas, according to the measurements 
of dukes, are overlain by more than 4000 feet of 
sedimentary strata before tlie upper or Bala volcanic 
series is reached. Three successive “ ash -beds ” 
constitute this upper series. Of these the lowest 
band, about 50 or 60 feet thick, was named a 
“ greenstone ash ” in contradistinction to a felstone 
o 
ash, and was not traceable for more than a short 
distance. Above it, after an intervening thickness 
of several hundred feet of sedimentary strata, comes 
a second and much more continuous band of tuff, 
known as the “ Lower ash-bed,” about 100 feet thick 
on the west front of the Berwyn range. Still higher, 
after an interval of about 1500 feet of slates, lies 
the “ Upper ash -bed,” which on the same line of 
section has a thickness of about 200 feet. This 
is the most persistent of all the volcanic horizons, 
for it can be followed continuously round the whole 
range of the Berwyns until it is overlain by the 
Carboniferous Limestone near Selattyn, a distance 
of not less than twenty-four miles. The same band, 
but much more feebly developed, has been traced 
through the faulted country on both sides of Bala 
Lake, where it formed a useful platform in the in- 
vestigation of the complicated geological structure 
of that area. Along the north side of the Berwyn 
Hills another thin band of tuff lies from 150 to 
200 feet still higher up in the series, and has been 
traced for a distance of aborrt twelve miles. The 
Bala limestone comes in about 800 or 1000 feet 
above the “ Upper ash-bed.” 
Besides the rocks now enumerated, the Survey 
maps show the intercalation of four or five sheets 
of “ greenstone,” which are represented as following 
with marked regularity the strike of the strata. 
Until these sheets have been more precisely examined 
it is impossible to decide regarding their true petro- 
graphical character, or to determine whether they 
are sills, or iuterstratified lavas, or include rocks of 
both these types. 
V. THE VOLCANOES OF ANGLESEY 
We now turn to another part of the country, about which much has 
