AND ON THE OKIGIN OF SOME EPIDOSITES. 743 



The mass in which these rocks occur forms a buttress, faulted 

 against the eastern flank of the Herefordshire Beacon, and has been 

 described as " altered Primordial " by the late Dr. Harvey B. HoU. 

 That they are of later Archaean or Cambrian age is probable. 



With regard to another specimen derived from this mass *, which 

 was described and figured in the paper which I last read before this 

 Society, mention was made of numerous minute granules which 

 were massed most closely along lines of perlitic fission in the rock, 

 and as to the nature of which I expressed considerable doubt at the 

 time. On re-examining the section and comparing it with those 

 now described, I am inclined to regard these small granules as epidote. 



Dr. W. S. Bayley, in a recent publication f, appears to have mis- 

 interpreted my statement on this subject, indicating that I considered 

 these granules to be topaz, whereas I " very doubtfully " referred a 

 few isolatad crystals to the latter mineral. It is quite possible that 

 these crystals are also epidote, as they give extinctions parallel and 

 at right angles to the length of the prism, which would occur in 

 orthodiagonal sections of epidote. 



From the foregoing observations it seems reasonable to think that 

 felsites resulting from the devitrification of obsidian, quartz-felsites, 

 aplite, arkose, and felspathic grits may, by the decomposition of the 

 felspathic constituent, pass, in the first instance, into rocks composed 

 essentially of quartz and kaolin, and that by subsequent alteration 

 of the kaolin in the manner already indicated, they may eventually 

 be converted into epidosite. This, however, I put forward as a 

 suggestion, not as a conclusion. According to the relative percentages 

 of quartz and felspar originally ]?resent in rocks, so, by alteration, 

 they may eventually become more or less epidositic, and, in cases 

 where the original percentage of quartz was low, a true epidote 

 may result. The three sections now exhibited show three difi'erent 

 grades of epidosition, although their original characters were probably 

 identical. 



Concerning the origin of epidosite, Xalkowsky says : — " One 

 frequently meets with rocks consisting of epidote and quartz, which 

 are generally termed epidosite ; in many cases these are, however, 

 nothing else than the last decomposition- and alteration-products of 

 basic anogenic and katagenic rocks. There are probably also original 

 epidote-rocks which belong to the quartzite family, and may be called 

 epidote quartzites " J. 



In the theory which I have here proposed it seems needless to 

 demand that the rock should have been originally of a basic character, 

 although I would in no way question the truth of Dr. Kalkowsky's 

 statement, for there may be more ways than one in which epidosites 

 are formed. In the case of the felsites from the Herefordshire 

 Beacon it is evident that the epidote was not an original constituent 



* " On the Eocks of the Malvern Hills/' Quart. Journ. Geol. Soc. vol. xliii. 

 p. 499. 



t " Summary of Progress in Mineralogy and Petrography in 1887," published 

 from monthly notes in the ' American Naturalist,' p. 1112. 



t ♦ Elemente der Lithologie,' p. 272 (Heidelberg, 1886). 



