1871.] HARKNESS AND niCKS ST. DAVId's PROMONTORY. 387 



Silica 78 



Alumina 16-5 



Lime, soda, and iron 55 



100 



The proportion of silica afforded by the foregoing analysis much 

 exceeds that which is obtained from rocks having a syenitic nature. 



These quartziferous rocks form an E.N.E. and W.S.W. course ; 

 and near the centre of this ridge is the city of St. David's. 



The arrangement of these rocks, which seem to be quartziferous 

 breccias, is rather indistinct. In the immediate neighbourhood of St. 

 David's, and also near Clegyr Bridge, about a mile E. from St. 

 David's, they have associated with them irregular bands of hard 

 greenish -coloured ashy-looking shales, considerably altered in cha- 

 racter, but in many instances possessing distinct traces of foliation. 

 Bands of this shale have also been met with in well-sinkings in St. 

 David's*. 



Differences prevail in the characters of the rocks which repose 

 upon the central ridge, those on the S.S.E. side presenting an asj)ect 

 somewhat different from those on the opposite side. This difference 

 seems to have resulted from faults which have brought various rocks 

 of the purple and green series into contact with the quartziferous 

 breccias. 



The quartziferous breccias on the S.S.E. have, for the most part, 

 resting upon them quartz rocks of a greenish-grey colour and 

 compact nature. These possess many of the features of horn- 

 stone, and they are intersected by closely approximated joints. 

 They have, however, a distinct S.S.E. dip at a high angle, usually 

 about 80° ; and in this position they are well seen in the neigh- 

 bourhood of Clegyr Bridge, where their strike is parallel to the 

 axis of the quartziferous breccias. 



On following the line of strike of the hornstones for a short 

 distance towards the W.S.W., these rocks are found to disappear. 

 At the ruins of Nun's Chapel, about a mile W.S.W. from Clegyr 

 Bridge, the compact quartz rocks are seen ; but immediately beyond 

 this all traces of them are lost, and a greenish flaggy sandstone 

 appears at Forth Clais Harbour resting against the quartziferous 

 breccias. 



The connexion between the quartziferous breccias and the greenish 

 flaggy sandstones at Forth Clais Harbour results from a fault, having 

 a throw-down on the E.S.E. side, by means of which rocks newer 

 than the compact quartz rocks, or hornstones, are broiight into con- 

 tact with the quartziferoijs breccias. At Porthlisky, a little to the 



* Since this paper was i-ead, I have been fortunate enough to find two or three 

 spots in the ridge wliere the bedding is distinctly shown, and where the shales 

 are seen alternating with the more massive quartziferous beds. The strike of 

 the beds is from N.W. to S.E., and hence quite discordant to the overlying 

 Cambrian series, in which the strike is from N.E. to S.W. This ridge, there- 

 fore, must represent a more ancient group of rocks than the Cambrian, oc- 

 cupying a position equivalent to that of the Laurentian group in Canada. — 

 H. Hicks. 



2e2 



