Hyland — On some Spherulitic Rocks from Co. Down. 435 



of that region are chemically and mineralogically undistinguishable 

 from the hypersthene andesites of Tertiary and yet more recent 

 age. 1 The volcanic eruptions which gave rise to these rocks must 

 have taken place in Lower Old Red Sandstone times. S. Allport 2 

 has shown that the devitrified perlitic rock of the Wrekin, which 

 is indubitably older than the Lingula Flags, and in all probability 

 is one of the later pre- Cambrian lavas, is as nearly as possible 

 identical, chemically, mineralogically, and structurally (except for 

 devitrification) with the perlitic obsidian of Hlinik, near Schemnitz, 

 and corresponds very closely with another post-secondary obsidian 

 from Hungary. The red felsite, which in North Wales is found 

 below the base of the Cambrian series, exhibits, in some localities, 

 a fluidal structure, and every indication of having once been a 

 true glass, and is chemically identical with the above rock from 

 Schemnitz ; while several of the lavas of the Ordovician series in 

 Wales are, except in this one regard of devitrification, not to be 

 distinguished chemically and microscopically from ancient rhyo- 

 lites, exhibiting fluidal or perlitic spherulitic structures. 3 Zirkel 

 was long ago impressed by the same similarity, when he stated 

 that many ancient felsite-porphyries cannot be distinguished 

 microscopically from recent quartz-trachytes ; 4 whilst Vogelsang 5 

 expressed himself inclined to the opinion that the felsitic ground- 

 mass represented the result of secondary devitrification. 



In the light of these considerations, and remembering (if we 

 still wish to make geological age a factor in nomenclature) that 

 the rocks may possibly be of Tertiary age, we may, with safety, 

 propose the name " Spherulitic Quartz-Trachyte " to denote the 

 rocks from Co. Down. 



1 Notes on the Cheviot Andesites and Porphyrites, Geol. Mag. 1883, vol. x., pp. 

 100, 145, 252. 



2 On Certain Ancient Devitrified Pitchstones and Perlites from the Lower Silurian 

 District of Shropshire, Q. J. G. S-, Lon., May 23, 1877. 



3 Bonney, T. G., On the Quartz -felsite and Associated Rocks of the base of the 

 Cambrian Series in N. W. Carnarvonshire, Q. J. G. S., Lon., 1879, xxxv., 309 : 

 also Presidential Address to Geol. Soc. 1885. 



4 Lehrbuch der Petrographie : Bonn, 1866 ii. p. 381. 



5 Philosophic der Geologie, &c. : Bonn, 1867, 144, et seq., 153, 194 ; also Archives. 

 Neerlandaises, tome vii., 1872. 



