258 rKOP. C. LLOYD MOKGAN ON THE PEBIDIAN 



are described by Dr. Geikie as the " prominent and massive porphyry 

 crags of Treginnis." That there is porphyry * I am not prepared 

 to deny ; but the main bnlk of the crags is fragmental — a sea- 

 green highly acidic andesite-tuff with quartz-porphyry inclusions of 

 fair size and in great numbers. Some " adinole " concretions occur 

 here ; and, commenting on one of my slides, Mr. Cole asks " Has 

 this rock been saturated (? with silica) so as to become so ' felsitic ' 

 and uniform in texture ? " The beds apparently dip northwards at 

 60°. But I was led to look with much suspicion at a northerly dip 

 of about this angle, since I frequently found it to be illusory and 

 the result of shearage. 



(y) The Red Quartz- Andesite. — At Ehoson and Trefeithan, on the 

 north side of Porthlisky Bay, at Pen Pedol and elsewhere, there are 

 in the " diabase "-tuffs large fragments of a red fluidal quartz- 

 andesite. They are for the most part angular. Near the landing- 

 stage at Carn-ar-wig there is a band or bed of similar lavaform 

 rock; and at Ogof Geifr a bed closely resembling it may be found. 

 Mr. Grenville Cole, to whom I submitted it, calls it " a beautiful 

 fluidal quartz-andesite (being ancient, a ' porphyrite ' of Germaii 

 authors). It reminds me of one at Elfdalen, Sweden, and many in 

 the Permian ' Bozen porphyries.'" 



(^) Volcanic Bombs. — The occurrence of large volcanic bombs in 

 rocks of such high antiquity is worthy of notice. Such a " diabase " 

 bomb, 3|- feet in diameter, may be seen in the tuff at the seaward 

 foot of the craggy boss just inland of Ogof Cadno. Others may be 

 found along this coast-line. They are rounded, vesicular, and often 

 radially fissured. 



(e) Pen-dal-aderyn and Pen-y-foel. — I am inclined to regard the 

 " diabase " masses at these two points as intrusive, but unquestion- 

 ably of Pebidian age. They may almost be looked upon as volcanic 

 necks. It may be that the extrusion of lava through them has pro- 

 duced the remarkable confusion in the neighbouring tuffs. Near both 

 these points, too, the relations of the more basic and more acidic 

 tuffs indicate local contemporaneous erosion. 



North of Pen-y-foel, towards Porthlisky Bay, there is another 

 great mass of " diabase " with well-marked trappean steps leading 

 down to the sea-level. This may be a massive lava-flow : but I am 

 more inclined to regard it also as an intrusive contemporaneous 

 neck. The same may be said of the exposure to the south of Porth- 

 henllys Cottage. 



d. Faidts and their Effects. — The long stretch of coast-line between 

 Ogof Golchfa on the north and Porthlisky bay on the south aftbrds 

 every facility for studying the faulting of the district. In tli<' 

 maps appended to this paper (Plate X.) I have entered all the faults 

 of which there seems to me to be satisfactory evidence. The only 

 displacement of any considerable magnitude is that which takes 



* Dr. Geikie, with great courtesy aud kindness, allowed me to examine aboiif 

 20 of his St. Davids slides, and among others one of the quartz- porphyry from 

 these crags. 



