222 THE OLDER ROCKS AT ST. DAVIDS. 



part lost." I was not able to iind anywhere in the district a 

 pebble of undoubted Dimetian, enclosed and worked up again in 

 the Pebidian series, but no doubt such must exist. This bed 

 just spoken of, is described in the later account by Dr. Hicks as 

 an agglomerate. It consists of blocks — some of them rounded as 

 if by water action — of pale felspathic rocks showing abundant felspar 

 crystals of primary origin as I take it ; these are mixed with a 

 more or less dark green paste of chloritic material. The bed is 

 very tough, the alkaline silicates so abundantly present in felspars 

 no doubt contributing by rearrangements to cement everything 

 together. Though the materials may be not quite in the state in 

 which they were laid down, owing to chemical changes, the bed is 

 far from coming under the definition of metamorphosis as restricted 

 above. There is a thick mass of it, and it would make a highly 

 ornamental stone from the contrasts of colour in it, but would be 

 very hard to work. Dr. Hicks has discovered among the included 

 blocks, some which weathered showed moniliform lines and beads ; 

 I am greatly indebted to him for a fragment. Under the micro- 

 scope it is seen to be a sphserulitic felsite porphyry, and has been 

 described by him with the aid of Prof. Bonney in his, as yet, 

 unpublished paper. If I touch upon it, it is only to draw attention 

 to the sphocrulitic porphyries described on the other side of the axis. 

 It is true the block given me by Dr. Hicks is not identical, absolutely, 

 with the rock of the School quarry 3 it is more perfectly sphocrulitic 

 and show.; .narkedly, fiuidal lines. There is, however, I contend, 

 the same composition and an analogous sphocrulitic structure. 

 There is a probability, hence, of their belonging to the same 

 period of eruption. In a thin slice its characters are as follows : — 

 Sph<£rulitic porphyry iv^gniQnt {vhyoWte oi Prof. Bonney) from 

 Pebidian agglomerate. The spheres are small but plainly seen 

 with a hand lens ; they are aggregated usually round a granular 

 centre of opaque material which does not polarise, the cross being 

 well seen in the spheres : some are free from the devitrified centre, 

 while others are almost entirely converted to amorphous granular 

 matter. 



