We now turn to another part of the country, about which much has been written and keen controversy has arisen. In the centre of Anglesey, among the rocks grouped together by the Geological Survey as "altered Cambrian," there occur masses of breccia, the probable volcanic origin of which was, so far as I know, first suggested by Professor Hughes.[222] Dr. Callaway regards them as pre-Cambrian,[223] while Professor Blake places them in his "Monian system."[224] When I went over them some years ago, I accepted the view that they are volcanic agglomerates.[225] Subsequent examination, however, has convinced me that notwithstanding their remarkable resemblance to true agglomerates they are not really of volcanic origin, but are essentially "crush-conglomerates," like those in the Isle of Man, so well described by Mr. Lamplugh.[226] [Footnote 222: _Proc. Camb. Phil. Soc._ vol. iii. (1880), p. 347.] [Footnote 223: _Quart. Journ. Geol. Soc._] [Footnote 224: _Op. cit._] [Footnote 225: _Presidential Address Geol. Soc._ vol. xlvii. (1891), p. 130.] [Footnote 226: _Quart. Journ. Geol. Soc._ vol. li. (1895), p. 563. See _Geol. Mag._ 1896, p. 481.] But though their present coarse, agglomerate-like structure is, I think, entirely due to the mechanical crushing of the rocks _in situ_ and not to volcanic explosions, it does not follow that the rocks which have been broken up do not contain evidence of volcanic action contemporaneous with their original formation. Obviously, pyroclastic materials may be subjected to deformation 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 having undergone 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 now pinched out, and now pressed up 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 crushing together of rocks _in situ_. A few miles further south at the beginning of the railway cuttings of Llangefni, green, red and purple slates and grits 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.