10 ME. F. RUTLEY ON THE SEQUENCE OF [Feb. 1 894, 



2. On tJie Sequence of Perlitic and Spuerulitic Structures : a 

 Rejoinder to Criticism. By Frank Rutley, Esq., F.G.S., 

 Lecturer on Mineralogy in the Royal College of Science, 

 London. (Eead November 22nd, 1893.) 



[Plate I.] 



In a paper on ' The Shap Granite and the Associated Igneous and 

 Metamorphic Rocks ' ' the authors, Messrs. Harker and Marr, did 

 me the honour to refer to certain statements made by me in the 

 year 1884 2 with regard to the microscopic structures of a perlitic 

 felsite, associated with the Coniston Limestone where it crosses the 

 northern end of the Long Sleddale Valley in Westmoreland. 



The opinion which I then expressed was that " spherules may 

 cause the devitrification of a rock after it has solidified and after 

 perlitic fission has supervened." A list of the structures, present in 

 the section examined, was also given in the following order of 

 sequence: — 1. Fluxion-bands. 2. Perlitic structure traversing these 

 bands. 3. Minute spherules constituting the whole rock, so far 

 as spherules can do so, 3 and passing through the perlitic fissures. 

 4. Subsequent fractures. 5. Formation of quartz-veins along these 

 lines of fracture. 



In the following year (1885) I again referred to this rock in 

 one of the Memoirs of the Geological Survey, 4 remarking that 

 " Spherulitic devitrification followed the development of the perlitic 

 structure." 



Messrs. Harker and Marr, after alluding to the microscopic draw- 

 ings of this rock published in Mr. Teall's ' British Petrography ' 

 (1888) pi. xxxviii. and to my own remarks, namely those which 

 have just been cited, made the following observation : — " The latter 

 author has expressed the opinion that the spherulitic structure is 

 here an effect of devitrification subsequent to the perlitic cracking ; 

 but we are unable to see that he has given any reasons for this 

 view. The practice of assigning a secondary origin to special struc- 

 tures in the older acid lavas has perhaps been pushed to excess in 

 some quarters. In the Westmoreland rhyolites there are traces of 

 perlitic fissures traversing rocks which are now microcrystalline, 

 and other appearances pointing to the alteration of an originally 

 glassy mass ; but we find nothing to suggest that the spherulitic 

 and allied structures are of formation posterior to the consolidation 

 of the lava : and the breaking up of the vitreous material of the 

 rocks examined seems to have been in many cases a chemical, not 

 merely a molecular change." 



Being averse to controversy, I might have allowed this statement 



* 1 Quart. Journ. Geol. Soc. vol. xlvii. (1891) p. 303. 



2 Ibid. vol. xl. p. 345. 



3 The devitrification of this rock is in some places microcrystalline. 



4 ' The Felsitic Layas of England and Wales,' p. 13. 



