374 MR. W. W. WATTS ON THE OCCURRENCE [Aug. 1 89 4, 



2. That the cracking in the crystals has taken place after the rock 

 hecame solid, as proved by the passage of cracks across the inlets of 

 matrix occurring in the crystals, and, indeed, cutting across every 

 constituent and structure of the rock, quartz, glass, felspar, micro- 

 lites, trichites, and flow- structure. 



3. That it is subsequent to the development of lithoidal structure. 



4. That the occurrence of perlitic structure cannot be safely 

 relied upon to prove that the rock has ever been in a glassy con- 

 dition, but that the lithoidal matrix is in certain cases original. 



5. That the glass of the rock has probably about the same 

 coefficient of expansion and contraction as the quartz, but not quite, 

 as there is always a little deflection in the cracks at the junction, and 

 always a tendency for them to bifurcate at the edge of the quartz ; 

 but that in the case of lithoidal (trachytic) structure there is 

 probably a considerable difference. 



This is not the first time that the occurrence of perlitic cracks in 

 crystals has been described, although the chief instances have not- 

 attracted much attention in England. MM. Eouque and Levy 1 in 

 Ib78 referred to two or three instances known to them : — " Dans 

 le groupe des porphyres quartziferes nous trouvons la roche de Per- 

 seigne, pres d'Alen^on, qui presentedes fissures spheriques developpees 

 autour et meme au travers des grands cristaux ;" and again, speaking 

 of the hornblende-andesites of the region south of Santorin, " Les 

 fentes de retrait traversent les grands cristaux." 



Mr. Rutley, in his paper on the vitreous rocks of Montana, 2 says, 

 " In the section to which our attention is now confined there are 

 plentiful examples of doubly-refracting crystals which are imme- 

 diately surrounded by perlitic cracks, but which do not, save very 

 exceptionally, transgress those boundaries ; " and again, " In those 

 few instances in which a perlitic crack passes through a crystal, there 

 is commonly another crystal developed by its side (p. 397)." 



The circular cracks seen in olivine have often arrested the present 

 writer's attention, and a description of them by Prof. J. P. Iddings, 

 noting the likeness to perlitic structure, will be found in ' The 

 Geology of the Eureka District, Nevada ' (1892), Appendix B, 

 pp. 387, 388, and pi. iii. fig. 11, Monographs of the U.S. Geol. 

 Survey, vol. xx. 



Slight traces of similar perlitic structures have been noticed in the 

 pitchstones of Donegal and Newry, and also in the porpbyritic 

 crystals in some of the igneous rocks of Charnwood. 



These instances would have rendered unnecessary any description 

 of the specimen from Sandy Braes ; but the chain of structures and 

 evidence seemed so complete as to merit a detailed description. 



1 Comptes Rendus Acad. Sci. vol. lxxxvi. (1878) p. 771. 

 3 Quart. Journ. Geol. Soc. vol. xxxvii. (1881) p. 391. 



