356 British Volcanic Rocks. | July, 
Lower Silurian volcanoes. Again, the traps of Limerick are 
regularly intercalated among the carboniferous limestones, and are 
consequently of the same age. 
II. By thus attending to the geological position of the strata 
with which a group of volcanic rocks is associated, we learn more 
or less definitely the era of eruption. And as the result of such an 
investigation, it is known that there are in the British Islands 
examples of lavas and ashes of many different ages, from the Lower 
Silurian up even to Miocene times. 
During the accumulation of the vast thickness of the Lower 
Silurian strata, there were active submarine volcanoes on the site of 
what is now North Wales ; and many of the more noted hills and 
valleys are formed in great part out of the old lava-streams and 
showers of ash. Snowdon is a striking example. That mountain 
is built up of several thousand feet of strata of volcanic ash, 
mingled especially in the upper part with sandy, calcareous, and 
argillaceous sediment. It is in truth a colossal monument of long- 
continued volcanic activity. That the volcanoes of that region were 
submarine and not terrestrial, 1s shown by the occurrence of 
marine fossils in the ashy layers, belonging to well-known species 
of the Caradoc or Balarocks. There is evidence that the volcanoes 
were active in more than one part of the Lower Silurian period. 
During the accumulation of the Llandeilo flags there was a 
vigorous group of submarine volcanoes in the district of Cader Idris, 
Aran Mowddwy, and Arenig Fawr. These died out, and after- 
wards, when the Bala beds were in the course of formation, the 
internal igneous forces broke out anew over the region around 
Snowdon.* 
In Ireland, also, between the north of Wicklow and Waterford 
Harbour, the Lower Silurian series abounds in felstones and ashy 
beds. 
The Old Red Sandstone of Scotland contains a great develop- 
ment of volcanic rocks. They form the chains of the Sidlaw and 
Ochil Hills, the Pentlands, and other groups. They are felspathic 
traps, ashes, and conglomerates, forming, by their decomposition, 
smooth green uplands and detached green conical hills. 
Throughout the central valley of Scotland, also, the car- 
boniferous formation is richly charged with traces of contem- 
poraneous igneous rocks. Indeed, during the growth of that for- 
mation the lowlands seem to have been dotted over with little 
* The reader who wishes to study the voleanic history of North Wales should 
read the works of Sir R. I. Murchison, and consult the elaborate maps of the 
Geological Survey, combining the results of long years of patient research by 
Ramsay, Jukes, Selwyn, Aveline, and other members of the Survey. The descrip- 
tive catalogue of the rock specimens in the Jermyn Street Museum will be found 
also very useful; but the great work on the subject will be Prof. Ramsay’s forth- 
coming Memoir on North Wales. 
