88 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



miiienfc feature on the flanks and crest of Caer Caradoc, shows 

 abundant finely banded flow-structure, often curved or on end, 

 while its bottom and upper parts are strongly amygdaloidal, the 

 cavities being occasionally pulled out in the direction of flow and 

 lined with quartz or chalcedony. Some of the detached areas of 

 eruptive rocks show the beautiful spherulitic and perlitic structures 

 first noticed in this region by M.Y. AUport. 



The breccias and tuffs appear to consist mainly of felsitic mate- 

 rial. In the coarser varieties fragm.ents of finely banded felsite 

 may be noticed, while the finer kinds pass into a kind of hornstone 

 (halleflinta), which in hand-specimens could hardly be distinguished 

 from close-grained felsite. In some places these pyroclastic rocks 

 are well stratified, but elsewhere no satisfactory bedding can be 

 recognized in them. Various other rocks which are probably intru- 

 sive occur in the ridge. At either end of the Wrekin there is a 

 mass of pink microgranite, while at Caer Caradoc numerous sheets 

 of " greenstone/' intercalated in the fine tuffs, sweep across the hill. 

 That some at least of these basic rocks are intrusive is manifest by 

 the way in which they ramify through the surrounding strata. 

 But others are so strongly amygdaloidal and slaggy that they may 

 possibly be true interbedded lavas, though it is difficult to under- 

 stand how such basic outflows could be erupted in the midst of 

 thoroughly acid ejections. Leaving these doubtful flows out of 

 account, we have here a group of undoubted volcanic rocks repre- 

 sented by acid lavas and pyroclastic materials, by intrusive bosses 

 of acid rocks, and by younger basic sills. The general lithological 

 characters of these masses and the sequence of their appearance thus 

 strongly resemble those of subsequent Palaeozoic volcanic episodes. 



The geological age of this volcanic group is a question of much 

 interest and importance in regard to the history of vulcanism in 

 this country. An inferior limit to its antiquity can at once be 

 fixed by the fact that, as originally pointed out by Dr. Callaway, 

 the quartzite which overlies the volcanic rocks passes under a lime- 

 stone containing Cambrian fossils in which Professor Lapworth has 

 since recognized Olenellus, Paradoooides, and other Lower-Cambrian 

 forms.' The eruptions, therefore, must be at least as old as the earlier 

 part ol the Cambrian period. But it is affirmed that the quartzite 

 rests with a complete unconformability on the volcanic rocks. If 

 this be so, then the epoch of eruption must be shifted much farther 

 back. 



The evidence adduced in favour of this great break appears to me 



