CHAP. VIII 
URICONIAN ERUPTIONS 
129 
later formations, while the sheared portions pass into epidiorites and true 
hornblende-schists. As in other regions where eruptive rocks have been 
crushed down and changed into the schistose modification, it is frequently 
possible to see groups of uncrushed cores roxind which, under severe 
mechanical stresses, the rock has midergone this conversion. Lines of 
movement through the body of the rock may lie detected by bands of schist, 
the gradation from the solid core to the hornblende- schist being quite 
gradual. The accompanying figure (Fig. 38) represents a portion of one of 
these crushed basic igneous rocks on the east side of Holyhead Straits. 
As in the Dalradiaii series of the Highlands, many, perhaps most, of 
these igneous bands are probably intrusive sills, but others may be 
intercalated contemporaneous sheets. They occur across the whole breadth 
of the island from the Menai Strait to the shores of Holyhead. 
Besides tliese undoubtedly igneous rocks, the green chloritic slates of 
Anglesey deserve notice. They are well - bedded strata, consisting of 
alternations of foliated fine grit or sandstone, with layers more largely 
made up of schistose chlorite. The gritty bands sometimes contain pebbles 
of blue quartz, and evidently represent original layers of sandy sediment, 
but with an admixture of chloritic material. The niannet in wliich this 
green chloritic constituent is diffused throiigh the whole succession of 
strata, and likewise aggregated into bands with comparatively little quartz- 
ose sediment, reminds one of the “ green schists ” of the Central Highlands 
and Donegal, and suggests a similar explanation. Taken in connection 
with the associated basic igneous rocks, tliese chloritic scldsts seem to me 
to represent a thick group of volcanic tuffs and interstratified sandy and 
clayey layers. If this inference is well founded, and if we are justified 
in gi'oupiug these Anglesey rocks with the Dalradian schists of Scotland 
and Ireland, a striking picture is presented to the mind of the wide 
extent and persistent activity of the volcanoes of that primeval period 
iu Britain.^ 
iv. THE UKICONIAN VOLCANOES 
Along the eastern borders of Wales a ridge of ancient rocks, much broken 
by faults and presenting several striking unconformabilities, has long 
been classic ground in geology from the descriptions and illustrations of 
Murchison’s Silurian Systems' The main outlines of the structure of 
that district, first admirably worked out by this great pioneer, were 
delineated on the maps and sections of the Geological Survey, wherein it 
^^’as shown that in the Longmynd an enormously thick group of stratified 
rocks, which, though unfossiliferous, were referred to the Cambrian system, 
rose in the 'very heart of the country; that to the east of these rocks lay 
' Mr. E. Greenly, late of tlie Geologic.al Survey of Scotland, has recently established himself on 
the Menai Strait for the purpose of working out in detail the geological structure of this interest- 
ing and complicated region. We may therefore hope that .some of the still unsolved problems 
presented by the rocks of Anglesey will before long be satisfactorily explained. 
^ See e.specially ehap. xix. vol. i. p. 225. 
VOL. I 
K 
