290 



A. GKEIEIE OJS" THE SUPPOSED 



Fig. 8. Section showing the Interstratification of Tuff and 

 Conglomerate above Lower Mill, St. David's. 



'-"jggg&u 



The conglomerate, as there exposed, is regularly interstratified with 

 tuff. The beds are vertical. On the left side of the little quarry as 

 we enter it, layers of fine reddish tuff (a) * are succeeded by a band 

 of quartz conglomerate (6) of the usual character. Parallel to this 

 conglomerate comes a band, about six inches thick, of fine tuff (c), 

 followed by ashy sandstone (d), which graduates into conglome- 

 rate (e). No more complete evidence could be desired of the perfect 

 inosculation of the conglomerate with the volcanic group. On the 

 coast at Nun's Chapel similar evidence presents itself. The con- 

 glomerate there contains some thin seams of tuff, and is intercalated 

 between a series of dull green agglomerates and tuffs and some 

 greenish shales and sandstones with layers of tuff. 



There is evidence that though volcanic action became dormant (or 

 at least exceedingly feeble) in the interval immediately succeeding 

 that in which the conglomerate was laid down, it reappeared at a 

 later part of the Cambrian period. Thus, about three miles east from 

 St. David's, on the road south from Pelin-canot, near Whitchurch, 

 beds of volcanic tuff may be seen overlying Cambrian grits. Again, 

 in a deep valley (Cwm Mawr) between Pointz Castle and Newgale 

 Bridge, on the coast, six miles east from St. David's, a coarse 

 volcanic tuff or agglomerate and ashy sandstone is interstratified in 

 beds which must be high up in the Lower Cambrian formation, 

 certainly far above any of the beds near St. David's. This rock 

 presents petrographical characters somewhat different from those of 

 the tuffs in the volcanic group above described (see Part II.). These 

 intercalations, which were noticed by Mr. Peach and myself as we 

 passed the localities, show that occasional eruptions took place 

 during the deposition of later stages of the Cambrian groups. A 

 more exhaustive search of the whole region would doubtless bring 

 to light other volcanic zones on different horizons, and enable us to 

 connect the volcanic episode of St. David's with that of the Lower 

 Silurian period in North Pembrokeshire. 



It follows, from the facts now detailed, that the volcanic group of 



* Red tuff of precisely the same character conformably underlies the con- 

 glomerate on the east side of the fold above Caerbwdy Mill. 



