Yol. 55.] GEOLOGY OP NOETHEEN Al^GLESBT. 657 



complete disruption is observed, the pinched-out bands being there 

 separated into distinct lenticles. A microscopic slide (N.A. 5Q) 

 cut from near the surface of one of the bands figured in the sketch 

 shows that the grit is considerably crushed and schistose. 



In the example just given the grit-courses do not exceed a few 

 inches in thickness, but disruption of thicker beds of Ordovician 

 grits has taken place at the eastern end of the district, south of the 

 bounding fault at Forth y Corwgl. In the face of the quarries 

 south of the Perth may be seen, lying at intervals in the Black 

 Slates, discontinuous masses of grit ; they are evidently parts of 

 bands similar to those seen a little farther north, which are clearly 

 interbedded with the slates. Similarlj'-shaped isolated bands of 

 conglomerate are shown by Prof. Blake in his figure of Perth Wen, 

 west side.^ 



North-west of Cemaes Pier, near Penrhyn, the fine slaty shales 

 contain numerous thin layers of tougher or more indurated material, 

 which, if followed for a few inches, are seen alternately to thicken 

 and thin in a lenticular way and quickly die out. This irregu- 

 larity of bedding seems best explained as an example, on a smaller 

 scale, of the disruption exhibited in Perth Newydd. Fig. 8 (p. 658) 

 shows this breaking-up in a small overfold. 



(/) The Crush-Breccias and Crush-Conglomerates. 



The completely disrupted strata known as ' crush-conglomerates ' 

 may be looked for, if we take the Manx examples as our guide, 

 along zones of powerful crushing, especially in areas where the soft, 

 fine-grained, slaty rocks alternate with tougher and more brittle 

 strata, such as grits and quartzites. Most of the crush-conglo- 

 merates which I am about to describe are indeed formed mainly of 

 lumps of hard quartzose beds in a finer matrix, and in some cases 

 they can be traced into areas where these harder portions form 

 recognizable interbedded bands. The occurrence and significance of 

 such beds in Anglesey seem to have been recognized to some extent 

 so long ago as 1880 by Prof. Hughes (although he does not 

 specify any localities) in his paper ' On the Altered Eocks of 

 Anglesey.' The following extract^ has most bearing on the subject : — 

 * When on a small scale nodules, or on a large scale masses, of hard 

 rock lying in compressible shale are subjected to contortion, the 

 shale is squeezed out over the harder masses, producing a kind of 

 fault all round between the harder and softer rock, and giving rise 

 to slickensides and similar phenomena, and often mineral changes 

 are set up along the parts thus more crushed. In the case of a 

 crumpled and gnarled shale in which there are thin laminse of 

 harder and softer beds, this unequal yielding must produce similar 

 results, and a kind of slickensides must pervade the whole mass.' 



Some of these rocks were described by Prof. Blake in 1888 as 

 volcanic agglomerates, and were included by him in his ' Disturbed 



^ Quart. Journ. Geol. Soc. vol. xliv (1888) p. 521, fig. 23. 

 2 Proc. Cambr. Phil. Soc. vol. iii (1880) p. 341 



a. J. G. S. No. 219. 2 u 



