OCCURRENCES OF TACHTLTTE. 307 



surrounding matrix possesses a perlitic structure *. The discussion 

 that has arisen as to the amorphous or crystalline condition of the 

 matrix of variolite may be explained if we admit the efficacy of 

 secondary devitrification, and if we regard the rock as having been 

 originally a massive tachylyte. The perlitic cracks, filled with 

 minerals of alteration, are admirably shown in one of the specimens 

 that I have to hand (PI. XI. fig. 6) ; and the works published on 

 the selvages of various " diabase " dykes, such as the recent paper 

 on Sordawalite by F. Loewinson-Lessing f, convince one that 

 developments of basic glass, and even coarsely spherulitic tachylytes, 

 have occurred on the margin of such intrusive masses in the past. 

 In Britain the search for variolite may not be vain ; and its dis- 

 covery will give us an indubitably basic rock to set beside the 

 " pyromerides " that are so weU represented in our Isles. 



I am much indebted to Profs. Judd and Bonney for kind assistance 

 in correlating the foregoing observations. Almost all the sections 

 have been prepared, and the chemical work has been carried out, 

 in the Geological Laboratory of the Normal School of Science 

 and Royal School of Mines. 



EXPLANATION OF PLATE XL 



[The numerator of the fraction expressing the degree of enlargement of an 

 object represents the magnifying-power of the objective with which it was 

 viewed.] 



Fig. 1. Junction of the intrusive sheet of Ardtun with the basalt above it. 

 Section showing bands of spherulites formed in the tachylyte on 

 and parallel to the plane of contact. X -^/^. 



2. Section of spherulites with polygonal outlines in the tachylyte of 



Ardtun. The rays showing the more fibrous structure are of 

 various tints of brown, the intervening areas being greyish. X -§-. 



3. Section showing small spherulites, cumulites, &c., in the tachylyte of 



the Quiraing. X ^^. 



4. Section of red-brown spherulites and green-mottled matrix in the 



altered tachylyte of Tollymore, County Down, x -%-. 



5. Section of the Ordovician Ash of Snead, near Bishop's Castle, showing 



fragments of altered basic glass, which contain porphyritic and 

 corroded crystals of felspar, x V^ • 



6. Section of pebble of variolite, from an old collection, showing per- 



litic structure ; the cracks being marked by alteration-products, and 

 the former glassy character of the rock being obscured by secon- 

 dary devitrification. X V • 



Discussion. 



Dr. SoRBY referred to the changes which had occurred since he 

 first took up the subject of rocks, and congratulated the Society on 

 the reading of papers like this one. 



Prof. Bonney also expressed his approval of the paper. He had 

 not been fortunate in finding tachylytes. He could corroborate the 

 Author's views as to the existence of these basic glasses in the 



* Ibid. p. 255 ; also Min^ralogie micrographique, planche xxiv. 

 t Tschermak's Mittheilungen, 1887, p. 61. 



