420 J. CLIFTON WARD ON THE MICROSCOPIC STRUCTURE OF 



Plate XVIII. 

 Fig. 12. Altered streaky ash, Base Brown, Borrowdale. Viewed with polarized 

 light, X 15. 



13. Altered ash, Hart Side, north of Glencoindale Head, x20. 



14. Altered fine-bedded ash, Great Gable, X 50. 



15. Highly altered ash, Bigghause End, Vale of St. John. Viewed with 



polarized light, X40. 



Wales. 



16. Felstone, Aran Mowddwy. Also represents the structure of fig. 15 



when viewed with plain light, X 40. 



17. Felstone, Aran Mowddwy. Viewed with polarized light, X 40. 



18. Felstone, Aran Benlynn, X 50. 



19. Upper part of Felstone, Llanberis Boute, Snowdon,x80. 



20. "Slaggy felstone," Llanberis Boute, Snowdon, X35. 



21. Fine-bedded ash, Glaslyn, Snowdon. Viewed with polarized light, 



X40. 



22. Ash, below the Aran f elstones, X 35. 



The figures were sketched and coloured direct from the microscope, by the 

 author. 



Discussion. 



Mr. Prank Eutlex stated that he had examined many of the 

 microscopic sections upon which much of the author's evidence was 

 based, and he believed that Mr. "Ward was, in the main, correct in 

 his conclusion that many of the rocks designated trap were inter- 

 mediate between dolerite and felstone ; for sections from one end of 

 the series presented the character of true basalt, while others from 

 the opposite end appeared, under the microscope, to be true felstone — 

 a variety of sections having also been cursorily examined which 

 passed, by almost imperceptible gradations, from the dolerites to the 

 f elstones. He considered that Mr. Ward had done as much in the 

 determination of these rocks as it was possible to do by the exa- 

 mination of merely ready-mounted sections — although, from this 

 method of investigation having been the only one at the author's 

 disposal while working in the field, he thought that the evidence 

 regarding some of the component minerals was scarcely satisfactory ; 

 and he adverted to the importance of examining sections of rocks 

 by other tests than that of mere microscopic scrutiny prior to the 

 final mounting of the preparations. He was inclined to regard 

 some of the ashes, of which sections were exhibited, as volcanic 

 ejectamenta, imbedded in a base much resembling devitrified, or 

 partially devitrified matter allied to pitchstone ; and he could not 

 agree with the author in regarding the fluxed character of this base 

 as a structure superinduced by metamorphism, at all events not in 

 the restricted sense of that term. 



Mr. Koch thought that the flow of materials around crystals may 

 be due to secondary actions. The effects of tension applied to 

 molten matter may produce appearances similar to those described 

 by the author. The study of the optical properties of crystals pro- 

 duced under tension would, he considered, materially assist in the 

 determination of the nature of the crystals in rock-masses. He re- 

 garded chemical analysis as not furnishing a sufficient test. 



