246 



[prof. C. LLOYD MORGAN ON THE PEBTDIAN 



/, probably originally continuous with this and with one of those 

 over the arch, has shifted the conglomerate so as to cause it to rest 

 on the bevelled edges of greenish-brown Cambrian sandstones. 

 These faults are not of any magnitude ; but they must be taken 

 into consideration for the understanding of the atratigraphical rela- 

 tions in this bay. 



Pig. 3.— St. Mn's Arch. 



aa. Pebidian. 



bb. Cambrian conglomerate. 



//■ 



cc. 

 dd. 

 Lines of fault. 



Cambrian, Caerfai beds. 

 Disturbed Caerfai beds. 



Matters are also complicated by the intrusion of massive quartz 

 porphyry (+ +) and diabase dykes (firm dark lines). Concerning 

 them, however, I have nothing here to say except that the intrusive 

 nature of the quartz-porphyries is unquestionable. 



Both at the east end and at the west end of the bay the Pebidian- 

 Cambrian junction can be studied. 



The little bay to the north-west of St. Non's Arch is not difficult 

 of access ; a steep grass slope, overshadowed by the face of the 

 porphyry-dyke, bringing one to the foot of the cliffs. Here the 

 porcellanites can be studied in detail and are seen to be fine-grained 

 sedimentary strata altered in situ. Pine lines of bedding are 

 beautifully shown on their weathered edges. Between the porcel- 

 lanite-beds are coarser, greenish, softer beds, which show diagonal 

 cleavage, the cleavage-planes here, as elsewhere, in the district 

 dipping to the N. at about 60°. Bands of brecciated tuff are here 

 and there interstratified. 



At low-water spring-tides it is possible to climb round under 



