GEOLOGY. OP ANGLESEY. 



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by hard grits. In the same neighbourhood arc bands of hornstonc, 

 similar to ordinary Pebidian types, associated with schist. 



(2) These rocks appear to pass up through less altered varieties 

 into slightly metamorphosed grits and shales, as seen south of 

 Llanfcchell and in localities to the east. 



(3) The alteration is not carried so far as is usual in the older 

 series. 



(4) In the undoubted slaty districts, as west of Cerrig Ceinwen, 

 the rock frequently undergoes partial metamorphism, and bands 

 occur which might almost pass for true foliated schists. 



(5) This area is scored with intrusive dykes. I do not suppose 

 that these are the cause of metamorphism ; but they indicate the 

 proximity of a source of heat, which may have been concerned in 

 the change. 



3. Llanfeehell Grits. — The altered rocks of Mynydd ATec-hell appear 

 to pass to the north into a series of green grits, well exposed round 

 Llanfeehell. This rock, has undergone much alteration, to which 

 probably the colour is due. Microscopic examination (Note 47, p. 234) 

 brings to light the important result that it is " almost certainly 

 derived from the older gneissic and schist rocks of this region of 

 North Wales." These grits are also seen on the strike to the south- 

 east near Bodewryd. 



4. Bhosbeirio Shales.- — -West of Rhosbeirio church, and in a quarry 

 near the farm, are very interesting exposures of felspathic rocks of 

 the true St.-David's type. These shales are soft, well laminated, 

 and fine-grained ; but here and there are thin seams of grit like the 

 variety just described. While the body of the rock is but slightly 

 altered, the gritty bands have a very schistose look. The prevailing 

 colours are pale green and purple ; but there are also some beds of a 

 soft yellow shale, like a common variety in North-western Anglesey. 

 The dip is to the north at 30°. Under this group, at Nant-y-cyntin, 

 slaty beds of a more altered character, intermediate between these 

 shales and the Cas-Clock schist, are seen. 



5. Sharply contorted Group south-east of Amlwch. — At Crogan 

 goch and on towards Llaneilian are exposed the remarkable rocks 

 described and figured by Prof. Ramsay. The foliated structure (Note 

 49, p. 234) of these slaty-looking beds is not very evident to the 

 naked eye. Similar rocks compose the tongue-shaped promontory 

 of Point iElianus ; but sometimes contortion is wanting. At the 

 base of the promontory the beds are gritty and partially altered 

 (Note 48, p. 235). 



6. Amlwch Slates and Cemmaes Limestones. — South of Amlwch, 

 near Crogan goch, the last group, with the intervention of an ashy 

 band, is succeeded by uncontorted pale-green chloritic slaty beds 

 (Note 50, p. 235), which are continued to the coast and all along the 

 west side of Bull Bay to Ogo' goch. The dip is steadily to the 

 north at a moderate angle ; and the thickness must be considerable. 

 Towards the west, on Porth wen, bands of limestone and quartz- 

 conglomerate come in ; and further west, on Cemmaes Bay, limestone 

 and quartz rock predominate over the slaty beds. I am not certain 



