412 



F. RUTLEY ON THE MICROSCOPIC STRUCTURE OP 



and triclinic felspars (oligoclase) and orthoelastic felspar, with some 

 magnetite in a fine microcrystalline matrix *. 



The rock No. 4 is possibly the crystalline representative of the 

 devitrified obsidians. The basalt (No. 3) is evidently an example of 

 the greenstone which occurs so extensively at this spot. Some of 

 the obsidians contain large spherical bodies sometimes an inch or 

 more in diameter, which are best shown upon weathered surfaces. 

 As these spherules are traversed by the fine bands which pass 

 through the rocks in which they occur, it seems reasonable to assume 

 that they have been developed subsequently to the solidification of 

 the rock. 



DESCRIPTION OF PLATE XXI. 



Fig. 1. Devitrified spherulitic rock from Beddgelert, showing general character 

 of spherules and matrix. X 18. 



2. Devitrified obsidian or rhyolite from Clogwyn d'ur Arddu, Snowdon, 



showing banded fluxion-structure. X 18. 



3. Devitrified spherulitic rock, No. 1. Beddgelert, showing spherules and 



spherulitic bands. X 6. 



4. Ditto, showing portion of a perlitic patch. X25. 



5. Ditto, showing parts of spherules at top and right side of field, and 



infiltered shrinkage-cracks in matrix. X 12. 



Discussion. 



The Chairman spoke of the value of the papers and the interest 

 of the agreement between rocks separated so widely in time or space. 

 Some of those described by the author might be paralleled by 

 instances from Arran and from the Auvergne. 



Dr. Sorby said the most interesting part of the paper was the 

 close relation of the structures described on the one hand to those of 

 granite, and, on the other, to those of slags. The relation in struc- 

 ture between slags and the older rocks was of great interest. 



Prof. Bonnet expressed his sense of the value of the paper. In 

 his opinion the only difference that could be maintained between 

 rhyolite and felsite (he thought f els tone should be used only as a 

 group term) was structure; he would understand by rhyolite a 

 trachytic rock in which a glassy base remained — by felsite those 

 in which the matrix was crypto- or microcrystalline. He knew 

 some of the rocks described by the author, and some remarkable 

 spherulitic rocks, one showing a structure just like that of Pont-y- 

 Gromlech on the east side of the Glyder. 



Rev. J. F. Blake asked about the formation of the crystals with 

 an inward growth, and whether large crystals did not indicate slow 

 cooling. 



Mr. Bauerman considered that the irregular strains in the obsi- 

 dian fragment described by the author might be due to contraction 



* The section cut from this specimen was hastily examined on the day upon 

 which this paper was read ; and I am not sure than it is not a clastic rock. 



