28 B. S. Butler — Pyrogenetic Epidote. 



Mr. W. H. Turner* considers epidote occurring in a fresh 

 soda-granite from California as probably original. Messrs. 

 Alfred E. Barlow and W. F. Ferrier,f describing epidote 

 occurring in Laurentian gneisses, consider it as primary, and 

 in a classification of the rocks, the micaceous gneisses are sub- 

 divided on the basis of primary and secondary epidote. These 



are described by the authors as follows : 



• 



"Biotite-epidote-gneiss. The combination of biotite and epi- 

 dote as the principal colored constituents forms a well-defined 

 rock-type which has been found to be remarkably constant over 

 large and widely separated areas The rocks are un- 

 doubtedly of irruptive origin, and are, in fact, foliated granitites, 

 thoroughly holocrystalline and granitoid, varying from coarsely 

 to finely crystalline." 



Under the description of the epidote they make the follow- 

 ing statements : 



"Next to the biotite, this is by far the most abundant of the 

 coloured constituents of the granitic gneisses and it also enters 

 largely into the composition of the more basic hornblendic ones. 

 In addition to the ordinary occurrence of the epidote as an altera- 

 tion product, we have also the strongest evidence that it exists in 

 a large number of cases as an original and important constituent 

 of the rock mass. 



The manner in which the perfectly fresh crystals, possessing 

 sharply defined outlines, occur inclosed by wholly unaltered bio- 

 tite in rocks which have been subjected to only a slight degree of 

 pressure, admits of no reasonable doubt as to their primary 



nature The crystals occasionally contain cores of a pleo- 



chroic brownish substance which is probably allanite, but no 

 thoroughly typical examples of that mineral were detected." 



As seen from the foregoing descriptions, in most of the 

 occurrences where epidote has been considered primary it has 

 been associated with allanite, the two exceptions being the 

 occurrence noted by Mr. Turner, where allanite is not men- 

 tioned as a constituent of the soda-granite, and that of Messrs. 

 Barlow and Ferrier, in which allanite is only rarely associated 

 with the epidote. 



The evidence of pyrogenetic origin adduced in these cases 

 has apparently not been entirely convincing. Several of the 

 recent text-books on petrography question the occurrence of 

 epidote as an original constituent of igneous rocks. Mr. 

 Waldemar Lindgren,^ in a recent paper: "Relation of ore 

 deposits to physical conditions," does not include epidote 



* Jour. Geol., vol. vii, p. 155. 



f Canadian Geol. Survey, vol. x, pp. 70-87, 1907. 



}Econ. Geol., vol. ii, p. 105, 1907. 



