66Q ME. C. A. MATLEY ON THE [Aug. 1899, 



geneous — whether they be soft and slaty, as between Bull Bay 

 and Forth Wen, or whether they be hard and resistant, like the 

 conglomerates and grits of Llanlliana headland and Torllwyn — 

 there is no marked disruption ; but whenever there are rapid alter- 

 nations of the soft and hard types, then disruption becomes more 

 or less pronounced, and on the horizons of such deposits come in 

 extensive zones of crush-breccias and crush-conglomerates. 



(v) Dykes in the crush-zones. — A number of subparallel 

 basaltic dykes cut through the crash-conglomerates at Penrhyn, 

 Llanbadrig, Forth Wnol, and Trwyn Fen-careg. They agree, on 

 the whole, in direction with the strike of the crushed beds, and are 

 clearly of later age than the movement ; for they keep a straight 

 course through the crush-conglomerate, and pass through it and the 

 associated great limestone-masses uninterruptedly. 



There are, however, traces of dykes which, being of earlier date 

 than the movement, have suifered equally with the rocks into 

 which they were intruded. I have found the relics of two of 

 these early intrusions. A few blocks of one of them lie in the 

 slaty crushed matrix at Trwyn y Fare. The other is to be found 

 east of the ' quartz-knob ' of Llanbadrig Foint. It lies mainly 

 in two lenticles surrounded by a broken quartzite-band and 

 some slaty beds, but other lenticles and pieces of it lie near, folded 

 and faulted among the other rocks. Not only have these dykes 

 been torn to pieces, but their internal nature and arrangement have 

 been altered. Almost all the structures and minerals have been 

 obliterated b)^ the movement, and their original composition and 

 texture can now only with diflSculty be recognized. Frof. Watts 

 states respecting them (N.A. 10, Trwyn y Fare ; N.A. 11 & Q5, 

 Llanbadrig) : — ' Originally they appear to have been porphyritic 

 basalts with a fine-grained groundmass. Their minerals are now 

 largely calcified, the ilmenite converted into leucoxene, and th& 

 porphyritic crystals are mere ghosts, smashed, filled up with 

 chlorite, and converted into aggregates of mica-flakes/ 



(vi) Method of formation. — The causes leading to the 

 production of crush-conglomerates have been already discussed 

 by Mr. Lamplugh in connexion with the Isle of Man examples, and 

 Anglesey serves to confirm his conclusion that they are mainly due 

 to the differential yielding of rocks when greatly compressed. The 

 Forth Newydd sketch (fig. 7, p. 656) shows that shale has been capable 

 of accommodating itself to the squeezing by flowing round the harder 

 masses ; but the grit-bands, although yielding and stretching to 

 some extent, have commenced to break, and the deficiency in exten- 

 sibility has been made up by the broken pieces moving away from 

 each other. The same effect has been produced, in the neighbour- 

 hood of this section, in the black shale itself ; for we find tougher 

 black shale forming lenticles and discontinuous layers in the softer 

 shale. Forth Newydd, too, shows that disruption can go on un- 

 accompanied by folding ; but folding has, without doubt, largely aided 

 the dismemberment of these rocks. Especially destructive to the 



