278 



A. GE1KIE ON THE SUPPOSED 



torn off a mass of conglomerate and associated tuffs. These rocks 

 have been so intensely indurated and silicified that the quartz pebbles 

 are hardly traceable on a fresh fracture, though they project more 

 evidently from a weathered surface. It is even difficult in places 

 to say precisely where the line between granite and conglomerate 

 should be drawn, so intimately are they welded together. The 

 former rock, still presenting its normal petrographical characters, 

 may be seen both underlying and overlying the involved portions of 

 conglomerate,- red shale, and fine tuff, the latter being altered into 

 a kind of hornfels or porcellanite. Yeins of granite penetrate these 

 altered rocks. Great numbers of diabase dykes traverse the granite ; 

 and some of them cut the Cambrian strata also. 



Fig. 6. — Plan of Junction of Granite iviili Cambrian Strata, 

 Ogof-llesugn, St. David's. 



The importance of this section in any discussion as to the nature 

 of the crystalline rock of the ridge south of St. David's cannot be 

 exaggerated. It is rather difficult of access, which may possibly 

 account for the absence of any description of it in the papers hitherto 

 published ; but it completes the demonstration that the rock, which 

 can be traced from St. David's to the coast south of Porth-lisky, is 

 an eruptive mass that has been intruded into the Cambrian strata. 

 The quartz conglomerate, here altered by the granite, is unquestion- 

 ably the same band which can be traced along the coast for fully 

 two miles eastward, and the greenish and reddish tuffaceous shales 

 are recognizably those that everywhere accompany the conglomerate. 



On the opposite side of this portion of the granite ridge the 

 eruptive mass comes into contact with the stratified rocks in the Bay 

 of Porth-lisky. Unfortunately, however, the actual junction is 

 obscured, on the cliffs by the decomposition of the schists that abut 

 on the granite*, and on the beach by the quantity of fallen blocks. 



The condition of the beach doubtless varies from time to time ; 

 but neither on the occasion of my first visit last September, nor on 

 the second examination five months later, could I trace the actual 

 contact of the two kinds of rock, though I followed them to within 

 a few feet of each other. The schists are, in some bands, much 

 indurated, passing into a kind of quartzite or quartz schist. The 

 crystalline rock of the ridge, as exposed along the cliff, presents 



* There may have been a sliif t at the line of junction here. 



