PRE-CAMBBIAN ROCKS OF ST. DAVEd's. 291 



strata named " Pebidum " by Dr. Hicks passes regularly upwards 

 into the fossiliferous Cambrian formations, from which it cannot be 

 dissevered, and with which it must be classed. There is no more 

 reason why a strong line should be drawn between the sedimentary 

 and volcanic groups here than in any other part of the geological 

 record of Great Britain. The intercalation of massive volcanic 

 groups is well known in the Silurian, Old Red Sandstone, Carbo- 

 niferous, and Permian systems. The Cambrian volcanic group of 

 St. David's is in every respect comparable to one of these, but with 

 an added interest from its high antiquity. It no more deserves to 

 be ranked as a separate formation with a distinctive name than the 

 thick group of tuffs and lavas in the Arenig Series of North Wales, 

 or that in the Old Eed Sandstone of Central Scotland. The term 

 " Pebidian," therefore, is unnecessary, and, having been assigned to 

 a group of rocks erroneously believed to be Pre-Cambrian, ought to 

 drop out of geological literature. 



5. Conclusion. 



At the beginning of this paper I stated my readiness to concede 

 that the maps of the Geological Survey, originally prepared thirty 

 or forty years ago, may now stand in need of correction, and that 

 in this spirit I went to St. David's, being fully persuaded that, in 

 regard to the map of that district in particular, very serious modi- 

 fications would be required. As the result of my resurvey, I find 

 that the true meaning of the volcanic group at the bottom of the 

 Cambrian strata there exposed, though partly recognized in the first 

 edition of the Map and Section, had been subsequently lost sight of, 

 these rocks having been erroneously renamed by the Survey " Altered 

 Cambrian " with intrusive sheets of " greenstone." I have freely 

 admitted this to be an important error. But it should be remem- 

 bered that the error was made nearly thirty years ago. Such, 

 meanwhile, has been the progress of petrography that a mistake of 

 the kind could not have occurred had the map been surveyed during 

 the last fifteen years. 



Again, were the area to be resurveyed now, we should not colour 

 as one continuous belt of intrusive rock the long strip of country 

 from the coast near St. David's north-eastward to beyond Llan- 

 howell. AVe should endeavour as far as possible to represent only 

 those portions of eruptive rock which are actually visible, or 

 unquestionably exist underneath the surface, leaving the intervening 

 spaces on the map to be coloured with the tint used for the general 

 stratified formation of each area. We should prefer to indicate in 

 this way that there are detached dykes and bosses along a certain 

 area of extravasation, rather than to mass the whole as one con- 

 tinuous belt. But this would be, after all, a question of detail or 

 style of mapping. The officers of the Survey were certainly correct 

 in regarding the crystalline rocks, which they named syenite and 

 felstone, as intrusive through the Cambrian strata ; and this is the 

 main question in the present discussion. 



