218 



C. CALLAWAY ON THE AECILEAN 



type is a true quartz-schist, sometimes passing into a quartzose 

 gneiss. As it was important to critically compare this rock with 

 the quartz rocks of Holyhead and Bodafon Mountains, I have had 

 four specimens cut ; and Prof. Bonner appends full descriptions 

 (Notes 25-28, p. 233). 



Grey Gneiss. From the section east of Ty Croes station, this 

 band may be traced to the north-east, forming the low ridge from 

 Tal y llyn to Gwalchmai. Passing to the east of Gwalchmai churchy 

 it is well exposed about Bodwinafawr, and on both sides of the 

 marsh (Cors Bodwrog) and pool. At Bodwrog church it is in contact 

 with granitoidite, the quartz-schist having in its turn been cut out. 

 In the ridge to the east of the church the gneiss is interstratified 

 with a highly crystalline limestone, or calcite-schist (Note 51, p. 236). 

 Prom the microscopic description it will be seen that this rock re- 

 sembles the quartzose limestone of Porth Trecastell ; and it is pro- 

 bably on the same horizon. The normal south-east dip is preserved 

 as far as the old Holyhead road, north of the marsh ; but near here 

 a change takes place, and black Palaaozoic shales come up to the 

 granitoidite. 



Bark Schist. This zone is not well seen north of the Holyhead 

 railway. The fault which limits it on the south-east gradually cuts 

 it out, so that east of Gwalchmai the slaty series approaches the 

 grey gneiss. The gneiss is exposed at Pen bryncle, dipping south- 

 east ; and half a mile to the south-east, near the milestone on 

 the Holyhead road, we find green slaty rocks with a north-east dip. 



III. Central Zone. 



This band extends across the island from sea to sea. It is the 

 most complicated part of this broken and contorted area ; but by 

 steady attention to the lithology, assisted by some stratigraphical 

 indications, clear results may be secured. 



a. Slaty Series. 



In following up the gneissic rocks towards Bodafon Mountain in 

 order to ascertain the relation between the gneissic and quartz 

 groups, I was surprised to find an area of slaty rocks between the 

 two. At Plas Llanfihangel dark gneiss and granitoidite dip to the 

 south-west : and a little further to the north, at Ma'n-addwyn, 

 felspathic shales dip as if they would pass below the gneiss. The 

 rock is of a typical St.-David's type, and unquestionably belongs to 

 the younger series. High up on the south-east slope of the moun- 

 tain are similar rocks dipping south-east. The dips in. both cases 

 being away from the quartz rock, it was natural to infer that the 

 shales rested immediately upon it. Further examination proved 

 that such was not the case. 



At the south-west end of the quartz ridge north-west of Ty'n- 

 llidiart the flanking rock is a sort of hornstone, banded pink and 

 green, of a common Pebidian type. Approaching the hill, the horn- 

 stone is seen to dip away from it to the south-west ; but surmounting 

 the low ridge made by the- hornstone, the beds are observed to roll 



