I2 PROCEEDINGS COTTESWOLD CLUB 1904 
Lapilli of felspar and felsite are abundant. Lavas are 
interbedded with the tuffs; but they are not so pro- 
minent as in Shropshire, and are sometimes of more basic 
composition. 
The Uriconian again appears in north-western Caernar- 
vonshire, forming two parallel bands, striking north-west 
and south-east, the more westerly extending along the 
eastern margin of the Menai Straits between Bangor and 
Caernarvon, the other emerging in the Llanberis district. 
The lower part of the series consists of massive flows 
of lava of the variety known as quartz-felsite, because 
the composition is so acidic that the silica is in excess, 
separating out as crystals and blebs of quartz. The upper 
Uriconian is fragmental. Professor Bonney describes 
felsitic grit, conglomerates interbedded with green grits, 
green breccias-and slates. A large proportion of these 
rocks are volcanic ejecta. The rounding of fragments in- 
the conglomerates and some of the grits indicates sub- 
aqueous deposition. At Bangor these beds are uncon- 
formably overlain by basal Cambrian strata, and near 
Llanberis the Lower Cambrian conglomerates rest upon 
the quartz-felsite, and contain rounded fragments of it. 
The Precambrian age of the volcanic group is thus 
distinctly proved. 
Rocks of Precambrian age also occur in Anglesey, and 
some of these bear a close resemblance to slaty parts 
of the Uriconian at Bangor and in Shropshire. Volcanic 
material is found in them; but the proportion is not 
great, and the strata must have been laid down at a 
distance from any volcanic eruptions. 
Returning to the Midland district, we find Precambrian 
volcanic rocks in Worcestershire, Warwickshire and 
Leicestershire. The Lickey quartzite, considered by the 
older geologists to be altered Silurian, has been determined 
by Professor Lapworth to be basal Cambrian, corresponding 
