BEVITRIFIEB ROCKS PROM BEDDGELERT AND SNOWDOK". 411 



Whether the banded obsidians, now felstones, are the " stratified 

 rocks of ambiguous appearance " is a point which can only be de- 

 cided by those who know the ground ; or perhaps they may be the 

 f* striped quartzose cornean " alluded to .as occurring between the 

 northern and southern masses of greenstone which constitute the 

 island. At all events, Sir Henry's description clearly shows that 

 lie had not overlooked these rocks, although he does not appear to 

 Jiave regarded them as lavas, or to have considered them of sufficient 

 importance to indicate their position on the Survey map. 



Rocks from SJcomer Island. 



1. Devitrified, banded obsidian. 



A compact greenish-grey felstone, petrosilex or hornstone. 



Under the microscope the banded structure is very clearly shown ; 

 and a minute spherulitic structure also occurs in places. The gene- 

 ral condition of the rock is microcrystalline. A greenish substance 

 is present in it, which appears to be chlorite. This rock is of much 

 the same character as the devitrified obsidian or rhyolite from Clo- 

 gwyn d'ur Arddu at the base of Snowdon. 



2. Devitrified, banded and spherulitic obsidian. 



A light greenish-grey to dark blackish-green rock. 



The specimen shows a weathered surface, upon which numerous 

 fine bands stand out in relief. The bands are much contorted. 



Under the microscope the banding is well shown, and the sphe- 

 rules are well defined and very numerous. A perlitic structure is 

 also clearly seen ; and greenish matter in many cases pervades certain 

 portions of the section. This green matter has in some cases a finely 

 granular structure, or else contains fine dust, and exercises a weak 

 depolarization when rotated between crossed Mcols. Spherules of 

 much larger size than those which constitute the bands are likewise 

 developed. They interrupt and appear to obliterate the fine sphe- 

 rulitic bands, which seem abruptly cut off by them. The crystal- 

 lization in these larger spherules is much more confused than that in 

 their smaller representatives ; and consequently they show no dark 

 cross between crossed Nieols. 



The structures in this rock are as perfect as any to be met with 

 in recent lavas. 



3. Basalt or andesite. 



An iron-grey rock with brown stains, compact in texture and 

 showing some minute glistening felspar-crystals. 



Under the microscope the rock is seen to be a finely crystalline 

 admixture of triclinic felspar prisms, granules of augite, and crystals 

 and grains of magnetite. Some isotropic matter seems also to occur 

 in the matrix. 



4. Quartz-oligoclase trachyte (?). 



A greyish rock, with small white porphyritic crystals. 



Under the microscope it is seen to consist of crystals of quartz 



