146 ON THE MICROSCOPIC STRUCTURE OF SOME WELSH ROCKS. 



some of the viridite resembles a pseudomorph after augite or horn- 

 blende. The rock is probably pyroclastic ; but the materials may 

 have been transported by water to some little distance. 



No. VIII. The same.— Fragments rather smaller, and rock more 

 speckled. Ground-mass much decomposed, with a considerable quan- 

 tity of a streaky green mineral, much of which, with crossed Nicols, 

 shows a fibrous structure and rather bright colours, probably akin' 

 to serpentine. In this are fragments of highly altered felspar, of 

 quartz (subangular), several lapilli of a devitrified trachyte, two 

 showing grains of quartz in situ, and some bits of a slaty rock; also 

 a few crystals of a dark-brown isotropic mineral, like an iron garnet 

 and a little (?) apatite. Like the last rock, probably pyroclastic \ 

 but perhaps the materials are a little rolled. 



No. IX. Banded Indurated Slate above the Agglomerate ; Bryniau., 

 Bangor.— Ground-mass finely granular, brownish, rather decomposed 

 (most of it dark with crossed Nicols), generally crowded with brokeu 

 ielspar^ crystals, with slightly banded arrangement, all very an- 

 gular m form, much decomposed, probably generally orthoclase, 

 several small lapilli of trachytic aspect, and a little magnetite or 

 llmemte. Like some of the finer banded slates of Charnwood Forest 

 this rock appears to be composed of volcanic dust. 



To conclude : IV. and VI. appear to have belonged to lava-flows, 

 and so render contemporary volcanic action very probable ; VII.-IX. 

 are, I think, conclusive on the point, the first two containing vol- 

 canic materials, even if they have been transported or mixed with 

 others, and IX. being inexplicable to me if not a true pyroclastic 



