236 



H. HICKS ON THE PRE-CAMBRIAN ROCKS OF ST. DAVID S. 



sections. Most of the harder compact conglomerates have been 

 dropped in the hill north of the Cathedral by a fault running in 

 a N.E. direction almost at the junction there of these beds with 

 the Dimetian axis. It is more than probable, however, that the 

 conglomerates on the west side of the St. David's valley are these 

 conglomerates repeated, as they have none of the usual appearances 

 of the Cambrian conglomerates. A tolerably thick series may also 

 be seen in section in the valley between Llanhowcll and Carvoriog ; 

 and the hard conglomerate beds creep up above the general surface 

 near the road-side between Llanhowell and Treglemaish, and near 

 Bryn close by the fault. To examine the series generally, however, 

 the first two sections mentioned, viz. Nun's Well and Caerbuddy 

 Valley, offer the best opportunities. The lower conglomerates are 

 well seen on the St. -David's side of the valley just above Clegyr 

 Bridge, and the fine compact porcellanite beds in some quarries 

 in the valley about midway between the bridge and the mill. 

 Immediately above the mill the conglomerates of the Cambrian rest 

 unconformably upon the porcellanites ; and masses of the latter may 

 here be seen in the conglomerates. At Nun's Well Cliff the porcel- 

 lanite beds are seen in section perfectly vertical, with the conglome- 

 rates again resting upon them unconformably. A thick dyke of 

 compact quartz porphyry cuts across the beds at this point, but does 

 not penetrate the Cambrian above *. This is shown in the following- 

 section : — 



Section at Nun's- Well Bay, extending from above the Well to the 



Coast. 



N.N.W. 



S.S.E. 



X>i'metlu.-n. ( 



TfO/J 



v 



Pebidinn. 



^) Cumbxlan 



* Prof. Judd reports as follows of these porphyries, the best specimens of 

 which are found in the Clegyr valley near Trepuet : — " The base shows little 

 besides zeolitic minerals and patches of ' viridite,' the latter probably derived 

 from some hornblendic constituent. But the quartz veins are numerous, and 

 of large size, ar.d exhibit the stone and (liquid ?) enclosures so common in 

 quartz-porphyries. Some of the large quartz grains are beautiful double 

 pyramids, with the intervening prism scarcely, if at all, developed. There are 

 large felspar crystals, some of which are very little altered ; and the latter are 

 certainly plagioclase. There cannot be the smallest doubt, therefore, that this 

 rock was a quartz- forphy rite. 



" The rock presents the most remarkable identity of character with the gold- 

 bearing quai-tz-porphyrite of Cstatye, near Vorospatak, in Transylvania, which 

 is an eruptive rock of Neogene age. It is probably the oldest quartz- 

 porphyrite yet noticed.'' 



