LAUKENTIAN VOLCANIC KOCKS. 
549 
stories, in a thickness of 350 feet. The bedded traps consist 
of feldspar porphyry, and other varieties ; and the inter¬ 
posed Llandeilo flags are of sandstone and shale, with trilo- 
bites and graptolites.^ 
The Snowdonian hills in Carnarvonshire consist in great 
part of volcanic tuffs, the oldest of which are interstratified 
with the Bala and Llandeilo beds. There are some contem¬ 
poraneous feldspathic lavas of this era, which, says Professor 
Kamsay, alter the slates on which they repose, having doubt¬ 
less been poured out over them, in a melted state, whereas 
the slates which overlie them having been subsequently de¬ 
posited after the lava had cooled and consolidated, have en¬ 
tirely escaped alteration. But there are greenstones asso¬ 
ciated with the same formation, which, although they are 
often conformable to the slates, are in reality intrusive rocks. 
They alter the stratified deposits both above and below 
them, and when traced to great distances are sometimes seen 
to cut through the slates, and to send off branches. Never¬ 
theless, these greenstones appear to belong, like the lavas, to 
the Lower Silurian 23eriod. 
Cambrian Volcanic Rocks. —The Lingula beds in North 
Wales have been described as 5000 feet in thickness. In the 
upper portion of these deposits volcanic tuffs or ashy mate¬ 
rials are interstratified with ordinary muddy sediment, and 
here and there associated with thick beds of feldspathic lava. 
These rocks form the mountains called the Arans and the 
Arenigs; numerous greenstones are associated with them, 
which are intrusive, although they often run in the lines of 
bedding for a space. “Much of the ash,” says Professor 
Ramsay, “ seems to have been sub-aerial. Islands, like Gra¬ 
ham’s Island, may have sometimes raised their craters for 
various periods above the water, and by the waste of such 
islands some of the ashy matter became waterworn, whence 
the ashy conglomerate. Viscous matter seems also to have 
been shot into the air as volcanic bombs, which fell among 
the dust and broken crystals (that often form the ashes) be¬ 
fore perfect cooling and consolidation had taken place.”f 
Laurentian Volcanic Rocks. —The Laurentian rocks in Can¬ 
ada, especially in Ottawa and Argenteuil, are the oldest in¬ 
trusive masses yet known. They form a set of dikes of a 
fine-grained dark greenstone or dolerite, composed of feld¬ 
spar and pyroxene, with occasional scales of mica and grains 
of pyrites. Their width varies from a few feet to a hundred 
yards, and they have a columnar structure, the columns be- 
* Murchison, Silurian System, etc., p. 325. 
t Quart. Geol. Journ., vol. ix., p. 170, 1852. 
