300 A. GEIKIE ON THE SUPPOSED 



felsite. The former group may have been derived from the explo- 

 sion of such rocks as the diabase-sheets of the district. The felsitic 

 tuffs have not been observed to contain any fragments of the micro- 

 crystalline quartz porphyries of St. David's. They have been 

 derived from true fine-grained felsites. There are various inter- 

 mediate varieties, due to the mingling in various proportions of the 

 two kinds of debris. 



3. They are marked by the presence of some characteristic fea- 

 tures of the volcanic vents of later Palaeozoic time, and in particular by 

 presenting the following peculiarities : — (a) minutely cellular lapilli 

 with spherical cells ; (&) lapilli with well-developed fluxion struc- 

 ture ; (c) lapilli consisting of a pale green serpentinous substance 

 resembling altered palagonite ; (d) lapilli derived from the destruc- 

 tion of older tuffs ; and (e) lapilli consisting of ejected crystals, 

 especially of felspars, sometimes entire, frequently broken. 



4. They frequently show that they have undergone metamor- 

 phism, by the development of a pale greenish micaceous mineral 

 between the lapilli, the change advancing until the fine tuffs occa- 

 sionally pass into fine silky hydromica schists. To this metamor- 

 phism further reference will be made in the sequel. 



I was unable to observe any evidence that the basic and siliceous 

 tuffs characterize two distinct periods of vulcanicity. From the 

 foregoing analyses it appears that some of the oldest visible tuffs 

 which are seen between Pen-maen-melyn and Pen-y-foel contain 

 only 48 - ll per cent of silica ; a specimen from Porth-lisky yielded 

 72-03 per cent, of that ingredient. Specimens taken even from 

 adjacent beds show great differences in the percentage of silica, as 

 may be seen in the analyses Nos. III. and V. 



This alternation of basic and siliceous fragmental materials has 

 its parallel in the neighbouring eruptive rocks, some of which are 

 olivine diabases containing only 45 per cent, of silica, while others are 

 highly siliceous quartz porphyries. But all the siliceous eruptive 

 rocks, so far as I have been able to discover, are intrusive, and 

 belong, I believe, to a much later period than that of the volcanic 

 group ; in no single instance did they appear to me to be true 

 superficial lava-flows. On the other hand, the basic eruptive rocks 

 occur both as contemporaneous sheets and as intrusive masses. The 

 presence of both siliceous and basic lavas in the Cambrian volcanic 

 reservoirs, however, is proved by the character of the tuffs. It 

 would appear that while the basic lavas were most abundant during 

 the volcanic period recorded by the rocks of St. David's, furnishing 

 the material for most of the fragmental eruptions, and pouring out 

 at the surface in streams of molten rock, the siliceous lavas did not 

 flow forth at the surface, but were copiously discharged in the 

 form of dust and lapilli. 



The rise of both basic and acid lavas at different periods in the 

 same or adjoining vents, so familiar in recent volcanic phenomena, 

 thus appears to have also characterized some of the oldest examples 

 of volcanic action. An interesting parallel may be traced between 

 the succession of events at St. David's and that which has occurred 



