220 
THE SILURIAN VOLCANOES 
BOOK IV 
been written and keen controversy has arisen. In the centre of Anglesey 
ainong^ the rocks grouped together by the Geological Survey as “ altered 
kambrian,” there occur masses of breccia, the probable volcanic origin 
0 which was, so far as I know, first suggested by Professor Hughes.^ Dr 
Callaway regards them as pre-Cambrian,^ while Professor Blake places 
them in his “Monian system.”® Wlien I went over them some years a^o, 
1 accepted the view that they are volcanic agglomerates.^ Subsequent 
examination, however, has convinced me that notwithstanding their remark- 
able resemblance to true agglomerates they are not really of volcanic oriain, 
but are essentially “crush-conglomerates,” like those in the Isle of Man°so’ 
well described by Mr. Lamplugh.® 
But though their present coarse, agglomerate-like structure is, I think 
entirely due to the mechanical crushing of the rocks m situ and not to 
volcanic explosions, it does not follow that the rocks which have been 
broken up do not contain eyidence of volcanic action contemporaneous with 
their original formation. Obviously, pyroclastic materials may be subjected 
to delormation and disruption as well as any other components of the earth’s 
crust, and may be equally converted into crush-conglomerates. And in 
Anglesey it can, I think, be shown that some of the rocks which have been 
broken up were originally tuffs and volcanic breccias. 
Throughout Anglesey the stratified rocks present evidence of haviiio- 
uiidergone very great compression, deformation and rupture. Thus at 
Llanerchymedd thick-bedded Lower Silurian grits, with their intercalations 
of shale, have been broken up by numerous small faults, and have been 
pushed over each other in large irregular blocks, the shales being no.w 
pinched out, and now pressed np into the interstices between the dislocated 
harder and more resisting grits. This condition of rupture may be regarded 
as one of the stages towards the formation of a conglomerate by the crush- 
ing together of rocks in situ. A few miles further south at the beo-innino- 
of the railway cuttings of Llangefni, green, red and purple slates and o-rits 
appear in a rather more crushed state, and immediately beyond these strata 
come the coarse breccias. Neither in their composition nor in their 
structural condition do these Llangefni strata appear to be marked off from 
the undoubted Lower Silurian rocks as parts of a different system. 
The railway cuttings at Llangefni reveal a series of rocks which appear 
to have been originally shales, with thin bands of siliceous grit. The 
argillaceous portions of this series are now green and phyllitie, and remind 
one of the finer parts of some basic tuffs among the older Palieozoic systems, 
hey include, however, pale flinty bands, such as might have been derived 
rom fine felsitic dust. The grits are for the most part fine-grained and 
iighly siliceous, but they include also coarser varieties with clear quartz- 
grains. The enormous deformation which these strata have undergone is 
^ /*)■«;. Ganib. Phil. Soc. vol. iii. (1880), ji. 347. 
^ Quart. Journ. Geol. Soc. » Op. cit. 
■* PresideniuU Address Gcol. Soc. vol. xlvii. (1891), p. 130. 
“ Qtiart. Journ. Geol. Soc. vol. li. (1895), p. 563. See Gcol. Mag. 1896, p. 481. 
