PLATE XXXVIII. 



PEELITIC FELSITE. 



TILL's HOLE, LONG SLEDDALE, WESTMORELAND. 



Fig. 1 magnified 50 diameters. Ordinary light. 

 Fig. 2 magnified 120 diameters. Nicols crossed. 



In the upper figure the perlitic structure is clearly shown. A secondary quartz- 

 vein is represented in the left-hand portion of this figure. The perlitic structure 

 proves the rock to have been originally a glass. The appearances represented in 

 the lower figure show that the glass has been devitrified. Two types of devitrifi- 

 cation may be recognized (a) the spherulitic and (b) the micro-crystalline. The 

 former is most extensively developed on the left, the latter on the right. The 

 spherulites are positive and give a well-defined black cross. This rock forms a 

 portion of a lava-flow associated with the Coniston limestone (Ordovician). 

 It has been described by Mr. RUTLEY (Q.J.G.S. Vol. XL., 1884, p. 340), who has 

 generously presented the author with the drawings from which this plate has 

 been prepared. 



