402 J. CLIFTON WARD ON THE MICROSCOPIC STRUCTURE OF 



west of the tSnowdon peak, though this is a good deal masked by a 

 network of yellowish-green, particles, with greenish and brown lines 

 running in the same direction as the cleavage. In polarized light, 

 when the prisms are crossed, the granular parts look like golden and 

 reticulated threads running all in. one direction, with here and there 

 a crack filled with a dark brown ferruginous mineral; and between the 

 threads are numerous drawn-out and often lenticular felsitic spaces 

 showing the characteristic coloured-breccia structure. The general 

 veined and reticulated appearance, however, is best seen when, the 

 analyzer being removed, a plate of selenite is placed behind the slide 

 and the polarizer is rotated. Under these conditions the granular 

 mineral shows itself strongly dichroic, the same colonr prevailing at 

 once over the greater part of the field of view, the complementary 

 appearing amongst it, but not so prominently. Thus, in one position 

 of the prism the reticulated veins may appear mostly red, with scat- 

 tered green spots and lines, in another position mainly green ; in both 

 cases the felsitic base forming the meshes is white and clear. 



When carefully examined, it is plain that, in all the examples of 

 felstone above described, the little particles and granules scattered 

 over the felsitic base are of the same mineral, presenting more or less 

 distinct dichroism ; and in this case of a cleaved felstone they have 

 been collected together along lines, and hence a more marked 

 dichroic effect is produced. I think, also, that there is little doubt 

 that the mineral is chlorite, seldom occurring crystallized, and often 

 being itself partly altered*. Before referring to the structure of some 

 of the slaggy felstones of Snowdon, it will be as well to describe that 

 of the felstone lying below the intrusive greenstone of Craig Wen, 

 near Capel Curig, since in some points it closely resembles the cleaved 

 Glaslyn rock. 



7. South of Llyn Cwlycl (Capel Curig).— -The rock under de- 

 scription is the lowest of the thin felstone beds just south of Llyn 

 Cwlyd. Its structure is very similar to the last, only that the 

 dichroic granules are even more numerous, and the felsitic base 

 appears only at intervals. There are small fragments of felspar 

 crystals disseminated throughout. The dichroism is as in the last 

 example, but not along lines, except in some parts, where there seems 

 to be a tendency to this veined structure. 



8. Llanberis Route, Snowdon, Slaggy Felstone (fig. 20). — The 

 general character of the lower part of the Snowdon Felstones is re- 

 presented in fig. 20. There is the same hazy and milky-looking 

 ground characteristic of the previously described felstones ; but the 

 scattered particles, many of which are slightly dichroic, are nume- 

 rous, and there is a great deal of the greenish mineral, forming 

 bands and streaks which frequently curve round the felspar frag- 

 ments. These last consist both of orthoclase'and plagioclase; the 



* The brown colour so frequently occurring with the green is probably due 

 to peroxidation of the iron protoxide ; or some of the green mineral may be 

 delessite or chlorophceite, which contain a larger amount of iron, and become 

 brown on exposure. The dichroism in the above case is generally well marked, 

 though faint, when the selenite is not used. 



