SOME ANCIENT AND MODERN VOLCANIC ROCKS. 403 



large fragment in the figure shows the coloured bands in polarized 

 light characteristic of the triclinic felspars. Prom the position of 

 this and other fragments, with their long axis at right angles to the 

 direction of the streaks, it seems probable that the state of fluidity 

 could not have been very great, or they would have been turned 

 round with the flow and made to lie with their longer diameters in 

 the direction of the bands. 



When viewed with a |-inch objective, the green bands are seen to 

 consist of an aggregation of the small light-green particles, scattered 

 over the clearer part of the base, and every here and there along the 

 bands the dichroism is decided, though partial at other spots. In 

 polarized light, the Nicols being crossed, the felsitic base shows the 

 usual coloured-breccia structure along lines separated by darker 

 bands and streaks of the green mineral, which certainly seems 

 to be chlorite. If chlorite, however, be in all cases a secondary 

 product, then the streaky character of the rock may be due, not 

 to its original flow as a lava, but to subsequent metamorphism ; and 

 the strong likeness between this and similar-looking rocks in Cum- 

 berland (fig. 11), which I cannot but regard as highly altered ash, 

 seems to render it possible that the Snowdon rock is also much 

 metamorphosed. 



Another specimen, showing a number of fine contorted lines very 

 like those of bedding, reveals very little indication of a banded struc- 

 ture, similar to the last, when cut at right angles to these lines. It 

 presents, however, in a most decided manner, the hazy structure as 

 shown in fig. 16, with numerous light-green particles scattered about 

 and sometimes collected into groups and lines. Many of these par- 

 ticles are decidedly dichroic. In polarized light, the characteristic 

 felsitic reaction is clear, and the only felspar crystals or fragments 

 of such to be seen in the slice are triclinic. 



c. Ashes of Snoivdon, Arenig, and the Arans. 



1. Glaslyn, Snowdon. — The specimens examined were taken from 

 near the faulted junction of ash and felstone just above and north of 

 Glaslyn. The ash presents a beautiful, finely bedded appearance out- 

 side, but has been altered into a compact, blue, felstone-like rock, just 

 similar to the altered ashes around parts of Scawfell in Cumberland. 



Yiewed with a I -inch objective, the structure is seen to be very 

 similar to that shown in fig. 14, from. Great Gable, Cumberland, and 

 fig. 16, from Aran Mowddwy — the former being a bedded, highly 

 altered, fine ash, and the latter a compact felstone. Upon the hazy 

 base are scattered many particles of the green dichroic mineral, 

 which are gathered together in groups in some parts, and fill cracks in 

 others. There are no distinct crystals or even fragments of crystals 

 of felspar in this case. "With crossed Nicols the coloured-breccia fel- 

 sitic structure is clearly shown, as in fig. 21 — the green mineral, where 

 its particles are closely aggregated, presenting a dark appearance. 

 As far as microscopic structure goes, there is no difference between 

 this specimen and the last, or the felstone of Aran Mowddwy. 



