668 MR. C. A. MATLEY ON THE [Aug. 1 899, 



-Ordovician. Some writers, as we have seen, have found a way out of 

 the difficulty by lumping all the rocks together as Bala. Prof. Blake 

 first definitely placed in the Ordovician the basal conglomerates and 

 overlying strata of Ogof Gynfor and Forth Llanlliana. 



The prosecution of detailed field-work has not yet revealed a full 

 or satisfactory sequence of the rocks in this area, though I am still 

 hopeful of succeeding in this task. Nevertheless, some remarks 

 on two of the principal rock-types and their relationship to the 

 Ordovician may be of interest. 



(a) Limestones. 



The limestones of the district are of two main types. One is a 

 fairly pure, blue limestone, effervescing freely with acid, and is the 

 only kind that is quarried for lime. Its largest exposures are at 

 Trwyn y Pare near Cemaes, and in the quarries near Llanlliana 

 Parm ; but it also occurs at Penrhyn, at Pig y Barcud, between 

 Llanbadrig and Cae-Owen, at the southern end of Forth Wen Bay, at 

 Carmel Head, and in patches at other places. It is sometimes a 

 laminated limestone with &imj black carbonaceous partings, but 

 often so crushed that the partings are almost obliterated and the 

 rock welded into a solid mass. 



The other type weathers characteristically as a light grey 

 rock, but is brown or dark bluish and often gritty internally. 

 Though it sometimes effervesces readily with acid, it is usually 

 impure, dolomitic, or siliceous. It is often found with the blue 

 limestone, and the two tj^pes appear to pass one into the other. It 

 sometimes weathers to a ferruginous-brown colour. Between Ogof 

 Gynfor and Forth Llanlliana there are strips of this impure lime- 

 stone running from the coast inland ; it occurs again on the south- 

 east side of Forth Wen Bay, in Forth Fadrig, on Penrhyn, and at 

 various other localities. I have had none of the limestones analysed, 

 but was told that the analysis of a rock from Forth Gynfor (Hell's 

 Mouth) showed it to be highly magnesian, as it yielded 61*15 per 

 cent, of carbonate of magnesia and only 21-40 per cent, of carbonate 

 of lime. This result, if trustworthy, could have been obtained, 

 I think, only from a quite exceptional piece of rock, the magnesia- 

 percentage being considerably higher than in pure dolomite. 



When the limestone retains its lamination the rock-structure is 

 traceable, and it is generally found to be highly crumpled, some- 

 times also to be imperfectly cleaved, so as to approach an imperfect 

 limestone-slate. Black shales sometimes occur with the limestone 

 and also reveal the bedding, as at Llanlliana Farm quarries, where 

 the dip is seen to be almost vertical. The obliteration of bedding 

 in the Trwyn y Pare quarry has been already noticed (p. 664), and 

 it is most obscure in the solid limestone of other exposures. Much 

 of it seems to be a cataclastic rock recemented by calcite and dolo- 

 mite or sometimes by quartz-veins. 



It was interesting to find 12 yards of limestone in a ' level ' 

 -driven into Mynydd-y-Garn near Ty'n-y-maen Chapel. This lime- 

 istone has no outcrop. 



