Mr. De la Beche ori the Geology of Southern Pembrokeshire. 3 



Associated with these aggregates are other rocks of a more obscure character, 

 yet so connected with the former, as clearly to be referrible to the same for- 

 mation. Of their real constitution, unfortunately, neither mineralogical cha- 

 racter nor chemical analysis affords any decisive evidence. They present a 

 fine-grained homogeneous paste, which often becomes porphyritic from cry- 

 stals of felspar, and more rarely of hornblende, imbedded in it : they occa- 

 sionally contain epidote, and perhaps also chlorite. I am inclined to consider 

 them as fine-grained aggregates of the three minerals so evidently displayed 

 in the above-mentioned crystalline varieties of trap, and to attribute to the 

 different proportions of these ingredients the subordinate shades of character 

 by which these rocks are diversified. Where the felspar may be supposed to 

 prevail, the result approaches to compact felspar*; where the quartz, we have 

 a regular hornstone, generally coloured, however, by minute green particles 

 disseminated through it, which probably consist of hornblende. The fine- 

 grained variety with an excess of hornblende has not hitherto been discovered 

 in the district. To certain of this family, in which hornblende predominates, 

 the French have given the name of Aphanite; but they would denote these 

 fine-grained rocks, generically, by the term pierre corneennej which so well 

 expresses their usual aspect. I shall naturalize, on this occasion, the term 

 Cornean, and shall apply to it the epithet Felspathic, or Quartzose, according 

 to the ingredient supposed to prevail in the rock. 



Besides the above-mentioned varieties of trap, there occurs one which has 

 the appearance of a conglomerate. The base is a black and very compact 

 trap, which contains grains of quartz, and seeming fragments of crystallized 

 felspar and of compact felspathic cornean. It is probably, however, a pseudo- 

 conglomerate, or concretional rock, originating in a peculiar and cotempora- 

 n'eous aggregation of its constituent parts f. 



* On felspar, as a constituent of similar rocks, see Dr. MaccuUoch on the Classification of 

 Rocks, pp. 480, 481. 



+ Trap-rocks, generically similar to those of Southern Pembrokeshire, are common to almost 

 all transition-districts. Thus in the northern part of the county, trap forms the entire collateral 

 chain to the north of the great range of Prescilly, and reappears on the line of the same chain in 

 many insulated points on either side of Fishguard. On the western side of Fishguard Bay greenstone 

 is found resting upon quartzose cornean ; and the insulated summits of a hill, remarkable for 

 having formed the camp of a French invading division, consist of greenstone. To cornean, as I 

 believe from the inspection of specimens, belong the rocks in Snowdonia, described as stea-schist 

 by Messrs. Woods and Phillips (see Annals of Philosophy for Nov. and Dec. 1822) ; genuine stea- 

 schist being in my opinion essentially connected with talcose slate, of which Snowdonia presents 

 no traces. Cader-Idris, and the range of the Arans and Aranigs, belong unquestionably to a 

 trap.formation ; as do the Wrekin and Caer.Caradoc in Shropshire, and the hills in the neighbour. 



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