DEVITRIFIED ROCKS FROM BEDDGELERT AND SNOWDON. 



409 



as I can judge from microscopic examination of the eruptive rocks 

 of the Llanberis Pass, the work which remains to be done is to fill 

 in in detail, on a larger scale, the lines now laid down upon the 

 1-inch map. 



I have already laid before this Society a description of a perlitic 

 rock which occurs at the top of the Glyder Fawr ; and I then ex- 

 pressed my belief that many more devitrified rocks would yet be 

 found among the fe] stones of palaeozoic age. 



The present paper is probably a very small contribution to this 

 list ; and it is to be hoped that, as the list increases, we shall learn 

 more precisely what a felstone is, and realize more fully what many 

 of the felstones once were. The vitreous lavas were probably closely 

 allied to trachytes. Other eruptive rocks, also occurring in the 

 Snowdon area, such as those of Llyn-cwm-y-ffynon and Pont-y- 

 Gromlech, are of a decidedly basic character. Thus we see that, as 

 it is now, so it was in the vastly remote period which we call Silu- 

 rian. The eruptive rocks of that age were both basic and acid ; and 

 their constituent minerals and structural features were similar to, 

 if not identical with, those which exist in, but do not specially cha- 

 racterize the rocks erupted at the present day. 



In view of these facts I think we may disclaim any power to 

 determine the age of a rock by its mineral constitution or structure, 

 and may protest, as Mr. Allport has done *, against the employment 

 of different names for similar or once similar rocks, which differ only 

 in point of age. 



Appendix. 



On the Eruptive Rocks of SJcomer Island f. 



Since the preceding paper was written I have examined some 

 specimens from Skomer Island, off the coast of Pembrokeshire. Por 

 many years they have remained undescribed in the collection of 

 rocks in the Museum of Practical Geology ; and my suspicion of their 

 true nature was first aroused by the close resemblance which one or 

 two of them bore to other devitrified lavas which I had previously 

 examined. Microscopic examination of these specimens shows con- 

 clusively that they are lavas of a once vitreous character. The sedi- 

 mentary rocks with which they are associated are regarded as 

 belonging to the Llandeilo or to the Bala series. 



Only a short account of the microscopic characters of these lavas 

 is here given, as they will be examined and described in greater 

 detail in the forthcoming edition of the official catalogue of the rock- 

 collection in Jermyn Street. 



The specimens about to be described consist chiefly of banded and 

 spherulitic rocks, now felstones, but once obsidians, the change being 



* " On the Microscopic Structure and Composition of British Carboniferous 

 Dolerites," Quart. Journ. Geol. Soc. vol. xxx. p. 565. 



t The following particulars are now laid before tbe Society by permission 

 of Prof. Ranisay, by whom also the specimens were collected. 



