Yol. 50.] PEELITIC CRACKS IK QUARTZ. 367 



23. Note on the Occurrence of Perlitic Cracks in Quartz. By 

 W. W. Watts, Esq., M.A., F.G.S. (Communicated by per- 

 mission of the Director-General of the Geological Survey. — Read 

 March 21st, 1894.) 



[Plate XVIII.] 



Contents. 



Page 



I. Introduction 3t>7 



II. Macroscopical Characters 3o8 



III. Microscopical Characters 368 



IV. Perlitic Structure in Quartz 370 



V. Conclusions 373 



I. Introduction. 



Since the publication of Mr. Allport's classical paper ' on the 

 devitrified pitchstones of Shropshire, in which he drew some very 

 cautious deductions from the presence of perlitic and other structures 

 in rocks of felsitic character, it has been customary to regard 

 perlitic cracks as an indication that the material in which they 

 occur has once been in a glassy condition. In 1893 I came across 

 a specimen (I 860), forming part of the old rock-collection of the 

 Irish Geological Survey, which raised grave doubts as to the safety 

 of this wide generalization. It was obtained from Sandy Braes, in 

 Antrim, and was a specimen of the ' pearlstone ' of Portlock. 2 When 

 cut it exhibited admirable perlitic structure, not only in the glassy 

 matrix of the rock, but also in the abundant porphyritic quartz- 

 crystals which it contained. 



Later on I found two specimens, collected by Mr. Rhodes, in the 

 English Survey collection (1 926, 927), which exhibited the same 

 characters, and one of them (1 927) was so well preserved as to 

 allow a thin slide to be made from it. It is this slide from which 

 the most satisfactory of the figures illustrating this paper have been 

 taken. 



The specimens were obtained from the quarries at Sandy Braes, 

 Ik mile N.E. of Tardree Mountain, in the northern part of the 

 great rhyolite mass which is shown on Sheet 28 of the Geological 

 Survey map of Ireland. I have reason to believe that the descrip- 

 tion of the petrological characters and field relations of these 

 rhyolites is in very good hands, so that I do not propose to give an 

 exhaustive description of the rocks ; it is sufficient for my purpose 

 to refer to the admirable descriptions of A. von Lasaulx. 3 



1 Quart. Journ. Geo!. Soc. vol. xxxiii. (1877) p. 449. 



2 ' Report on the Geology of Londonderry,' 1843, p. 212. 



3 Journ. Roy. Geol. Soc. Ireland, ser. ii. vol. iv. (l877j p. 227. and Tscher- 

 mak's Min. u. Petrogr. Mittheil. vol. i. (1878) p. 410. 



2c 2 



