Tol. 55.] THE GEOLOGY OE NOETHEEN ANGLESEY. 659 



Volcanic Group.' He remarks upon their unusual features, in some 

 localities especially, as in Forth Wnol, where ' there is wild con- 

 fusion in the rocks. The most remarkable feature is the occurrence 

 ■of great quartz-lumps, which are of all sizes and shapes, and lie 



promiscuously in agglomerates of slates, grit, and dust They 



must have been in existence previous to the formation of the 

 agglomerate, which we cannot here refer to the action of a crush- 

 fault In the headland of Pen-y-parc we get another agglo- 

 merate of quartz-lumps and ash.'^ 



In 1896 Sir A. Geikie, in a short communication to the British 

 Association, announced ^ that he was convinced, in the light of the 

 Manx examples, that these breccias are crush-conglomerates, pro- 

 duced by the breaking-up of stratified rocks (maiuly grits and 

 slates) in situ. The limestone-masses he refers to the disruption 

 of some of the limestone-bands of the neighbourhood. He, however, 

 points out that the strata which have been disrupted are largely 

 composed of volcanic detritus, so that the arguments he originally 

 based on the volcanic nature of these beds are not disturbed. 



Prof. Blake has expressed his inability to concur in this ' crush- 

 conglomerate ' theory, mainly on the ground that the Anglesey 

 agglomerates are composed of ' a variety of rocks not like those of 

 the immediate neighbourhood.' ^ 



(i) Trwyn Pen-car eg. — The example of a crush-conglomerate 

 that will be first noticed is to be found on Trwyn Pen-careg, a head- 

 land east of Cemlyn Bay, and it is selected for first description 

 as it occurs here in its simplest form and offers an intermediate 

 stage between the broken grit-bands of the Ordovician of Porth 

 Newvdd and the more complicated disruption to be seen west 

 •of Cemaes. Immediately north of Trwyn Pen-careg lies Cerig 

 Brith, with which the headland is connected by a shingly beach 

 when the tide is low. The promontory of Trwyn Pen-careg itself 

 terminates in a small cliff of crush-conglomerate of a somewhat 

 rudimentary character. It consists of greenish-grey slaty and gritty 

 beds, such as are seen on Cerig Brith, but broken into fragments. 

 Most of the pieces are angular, but some are rounded sufficiently to 

 be considered ' pebbles,' and their surfaces are covered by a glossy 

 sericitic mineral developed by the shearing. The ' pebbles,' or 

 authiclasts as they may be termed, are mostly small, one of the 

 largest noticed measuring 15 x 6 X 9 inches ; but much larger masses 

 of almost unbroken beds lie among the brecciated fragments. Some of 

 the purple slaty bands of the locality are included in the crush-zone. 

 Pieces of the purple rock lie among the green fragments, and by the 

 contrast of colour emphasize the fragmental character of the mass, 

 but on the whole the purple ' pebbles ' are localized in definite parts 

 • of the breccia, thus showing that the amount of relative displace- 

 ment has been small. 



The area of disruption is narrow and short. South of it we 

 find similar, much contorted, and overfolded greenish and purple 



1 Quart. Journ. Geol. Soc. vol. xliv (1888) pp. 517-518. 



2 Geol. Mag. 1896, p. 481. 3 jf^id, p. 570. 



2tj2 



