PEE-CAMBRIAX EOCKS OP ST. DAVID 8. 



277 



higher beds resting on his "Dimetian" ridge*. At every section, 

 instead of lying upon the granite unconformably, they plunge 

 beneath it. The general disposition of the rocks in this part of the 

 valley is expressed in the accompanying diagram (fig. 5). It will 



Fig. 5. — Sketch Plan of the Disposition of the Rocks in the Allan 

 Valley at Porth-clais. 





be evident from this map and from the foregoing description that 

 the relative positions of the two rocks cannot be accounted for by 

 faulting f . At Porth-clais the actual terminal curve of the granite 

 projection can be traced across the bed of grit through which it has 

 risen (fig. 4). These junctions are characteristically those of an 

 eruptive mass. 



But the most important junction of the granite and Cambrian 

 beds is that which has been cut by the sea in the range of cliffs 

 between Porth-lisky and Porth-clais at the little inlet of Ogof- 

 llesugn (fig. 6). The granite, which extends continuously eastward 

 from Porth-lisky, abruptly ends off, and is succeeded at once by 

 vertical sandstones and shales, which are truncated by it nearly at 

 a right angle?. On the seaward face of the cliff the granite has 



* Quart. Journ. Geol. Soc. vol. xxxiii. p. 231 (1877). 



t That there may have been some local slipping along the boundary-line 

 between the granite and the rocks it has invaded is not unlikely. In the Allan 

 valley the faults would need to be reversed ones, and to wind about so as 

 precisely to counterfeit the boundary-line of an eruptive rock. This subject is 

 further referred to in Part II. (p. 310). 



\ This locality is referred to by Dr. Hicks as a line of fault ; indeed, in his 

 map (Quart. Journ. Geol. Soc. vol. xxxi. pi. viii.), as already stated, he makes 

 the boundary -line between the two rocks everywhere a fault. I have admitted 

 that, along the flanks of the granite, occasional local slips may have taken place ; 

 but the visible sections prove that no continuous or important faults occur 

 there. Possibly some slight displacement may have taken place at Ogof-llesugn ; 

 but the mass of conglomerate is imbedded in the granite. It should be noted 

 here that, the section described in the text is the same as that already referred 

 to in the citation from Sir A. 0. Ramsay's early MS. report as affording the best 

 evidence of the intrusive nature of the igneous rock. 



Q.J.G.S. No. 155. y 



