222 



C. CALLAWAY OJN T THE ARCHiEAN 



ridge are a felspathic breccia and a compact felspathic rock like 

 hornstone ; but both varieties are quite distinguishable from any 

 other rocks in Anglesey. At the east end of the hill, by the Pen- 

 sarn chapel, the breccia is well exposed. The fragments, some of 

 which are a yard in diameter, are a very compact felstone ; and the 

 matrix is a sort of greenish ash. The rock, as a whole, reminded 

 me of the compact breccias at Clegyr Bridge, St. David's. Prof. 

 Bonney has since examined a slide ; and he is of opinion that it 

 exhibits both perlitic and fragmental structure and is in all proba- 

 bility a rhyolitic ash. The dip of the series is to the north, agreeing 

 with that of the adjoining slaty group. About 100 yards to the east 

 of the chapel, and therefore on the same strike, is a knoll composed 

 of a pale-green and purple rock, very compact, almost like jasper. 

 Under the microscope (Note 53, p. 236) it is seen to be a trachyte, 

 probably from a lava-flow, and is, in Prof. Bonney's judgment, as 

 modern in its appearance as the rock composing the enclosed pebbles 

 near Bodorgan. The purple variety is not unlike some of the flinty 

 felstone common in the Precambrian of Shropshire. From litho- 

 logical characters the Archaean age of this group appears not 

 improbable, though it must be conceded that it does not resemble 

 very closely the volcanic group south of Bangor. Field work throws 

 little light upon the problem. On the north side the ridge is bounded 

 by a fault ; and at the base of the southern escarpment black shales 

 seem to pass conformably under the volcanic group. The two rocks 

 appear within three or four feet of each other, dipping in the same 

 direction into the steep face of the hill. There are, however, no 

 signs of a passage between the black shales and the compact felspa- 

 thic rock. The appearance of conformity might be produced by 

 faulting, accompanied or followed by lateral pressure ; and there are 

 abundant proofs of both in the district ; but, under the circumstances, 

 I hesitate to include this group in the slaty series. 



2. Chloritic Schists of Mynydd Mechell. — This group is the " foli- 

 ated grit" of Ramsay. In the field it has the appearance of a 

 quartzose grit, with chlorite covering lamination surfaces ; but Prof. 

 Bonney, after microscopic examination, regards it as a true schist 

 (Note 45, p. 234) of the Holyhead type, though he admits that the 

 chloritic constituent is " rather minute." These rocks are much 

 contorted, and are frequently penetrated by dykes of felstone and 

 dolerite running with the strike. At a higher horizon, at Cas 

 Clock and west of Rhos-y-pill, the rock is an undoubted schist with 

 silvery lustre (Note 46, p. 234). Some of the strata north-east of 

 Paris Mountain are not unlike this schist, as may be seen at Cerrig, 

 on the Amlwch road. 



It is important to observe that Prof. Bonney agrees with me in 

 recognizing (Notes under C, p. 234) a difTerence between these 

 schists and those of the gneissic districts ; but in such a broken 

 country it is difficult to prove that these rocks should go with the 

 slaty series. My reasons for so associating them are the following : — 



(1) The rocks are not uniformly foliated. To the west of Cas 

 Clock, for example, chloritic schist is overlain in the same quarry 



