310 C. U. KEYES — EPIDOTE IN ERUPTIVE ROCKS. 



almost universally as never forming a primary constituent of eruptives, 

 or Archean masses, while as a characteristic ingredient it is abundantly 

 developed in metamorphic gneisses, schists and phyllites. It is also a 



common product of both acid and basic rocks containing feldspar. The 

 occurrence of epidote in acid eruptives has occasioned considerable 

 discussion. Among the earlier references may be mentioned certain 

 papers of Becher* and Blomstrand,f while the principal allusions to 

 the subject during the past decade have been made by Tornebohm,J 

 Geikie.§ Rosenbusch,|| Hobbs,^ and Adams.-* 



The general consensus of opinion as derived from the literature 

 referred to has been against the idea that the epidote was original in 

 any of the cases mentioned. Hobbsjf who was the first to study the 

 Ilchester (Maryland) granite, was inclined to believe that the epidote 

 was of metamorphic origin. Very recently Adams|| has investigated 

 some epidote-bearing granites from Alaska, in which the mineral alluded 

 to is thought to result from the recrystallization of certain of the rock's 

 constituents after the original solidification of the mass. The epidote 

 is regarded as "having grown into the surrounding minerals b} T first 

 sending out little arm-like extensions from its substance which subse- 

 quently met one another, in this way including some of the foreign min- 

 erals, which may or may not finally disappear " (page 349). Parallel 

 growths of allanite and epidote are explained by the former being- 

 regarded as " a primary mineral around which the epidote would nat- 

 urally crystallize, if any were developed in the rock, the two minerals 

 being isomorphous " (page 350). 



In his " Contributions a l'etude des gneiss a pyroxene et des roches a 

 wernerite"§§ Lacroix has figured and described some interesting occur- 

 rences of isomorphous growths of epidote and allanite in the amphibolic 

 gneiss of Geffren-en-Roscoff. They are considered analogous to Ilches- 

 ter examples with which he has compared them. These growths are 

 also reported from certain rocks of Finisterre, Norway, and Waldviertel, 

 the epidote in all these instances being regarded as primary (page 353). 



The association of certain of the allanites and epidotes in the granites 

 of Maryland is so intimate that there can be but little doubt that both 



♦Ueberdas Mineralworkomen in Granite von Striegan, u. /.. w. (Boslau). 

 tOefvers at', akad. Forhandl., no. '.'. 1854, p. :»6. 

 JGeol. For. i Stockholm Forhandl., vi, 1882, p. 245. 

 gQuar. Jour. Geol. Soc . vol. xxxix, p. :a 1. 



Mik. Phys., i Band, L885, p. 498. 

 * Johns Hopkins University Circulars, no G5, 1888, p. 70; also Am. .lour. Sri.. v<.l. xxxviii. 1889, 

 pp. 223-228. 



** Canadian Record Science, 181)1, pp. :;ll - 18 



tt Loc. fit. 



XX Loc. fit. 



g§Bul. Soc. min. de France, torn.' xii. 1889, p. 139. 



