744 ON" PEELITIC FELSITES, AOT) ON THE OEtGIN OP SOME EPIBOSITES. 



of the rock, since it lias been chiefly developed along fissures and 

 perlitic cracks, and the latter are known to have been formed during 

 or subsequently to the solidification of the rocks which they traverse. 

 It is also clear that in these instances the rock was originally vitreous, 

 and unaltered vitreous rocks never contain epidote, so far as I am 

 aware, although it is by no means uncommon in their devitrified 

 condition. 



EXPLANATION OF PLATE XVII. 



Fig. 1. Epidositic felsite (The Rabbit Warren, Herefordshire Beacon), showing 

 perhtic structure, marked by the development of epidote along the 

 perlitic cracks. X 25 linear, ordinai'y transmitted light. 

 2. Another portion of the same section, X 120 linear, showing traces of 

 perhtic structure. 



Discussion. 



The Peeszdext wished to hear the opinion of chemists as to 

 whether any solution acting on so inert a substance as kaolin could 

 convert it into epidote. 



Mr. Teall congratulated the Author on having carried the history 

 of the vitreous rocks a stage further. He could confirm his obser- 

 vations as to the frequency of epidote in perlitic rocks, and believed 

 it had been largely developed during the alterations of the old 

 vitreous rocks ; but he would not venture to express an opinion on 

 the process by which the epidote was formed. 



Mr. W. P. Blaee noted the frequent occurrence of epidote in the 

 Triassic rocks of IS^orth America, where it is sometimes extensively 

 interstratified with and formed in sandstones, as in the Sierra 

 Nevada. 



Mr. Baueehan commented on the way in which epidote was 

 formed throughout the mass of the rock, and suggested the bearing of 

 the matter of this paper on the question of the formation of such 

 masses of epidote as that of Lake Superior, which may also have 

 been formed, though on a large scale, in fissures. Epidote also 

 occurs in the same way in Cyprus. 



Mr. Cole asked whether, seeing that even pyroxenes occur in 

 many trachytes, it was not possible that the original rock contained 

 sufficient lime to account for the formation of the epidote by secon- 

 dary decomposition. He referred to the observations of M. Levy 

 on variolite of the Durance, where the perlitic cracks are marked 

 out by crystalline granules. 



The Atjthoe admitted the uncertainty connected with the precise 

 mode of formation of epidote, and stated that, so far as he knew, 

 epidote had never yet been produced artificially ; he was, however, 

 puzzled to imagine the extensive percolation of water through 

 the rocks without conversion of the felspar into kaolin. AVith 

 regard to Mr. Cole's question, he was inclined to think that the 

 rocks were practically devitrified obsidians, and he did not think 

 there was any evidence of the original existence of pjToxene in 

 them, although it was abundant in other rocks in their immediate 

 vicinity. 



