OCCURRENCE OF EPIDOTE. 321 



Epidote, like zoisite, is rarely if ever a pyrogenic constituent in igneous 

 rocks. It is, however, a secondary constituent in all varieties of metamor- 

 phosed igneous rocks, whether plutonic or volcanic, whether lavas or tuffs. 

 It is an allogenic constituent of the sedimentary rocks, and it extensively 

 develops in the sedimentary rocks as a secondary product. It is particularly 

 likely to form in rocks rich in calcium and iron, whether igneous or sedi- 

 mentary; and thus is especially abundant in those metamorphosed igneous 

 rocks which contain ferriferous varieties of pyroxene and amphibole, and 

 in metamorphic sedimentary rocks which contain a considerable amount 

 of calcium, as, for instance, calcareous schists and gneisses and marble. 

 In the metamorphosed rocks epidote occurs alike in those which have a 

 strongly developed schistose or gneissose structure and in those which have 

 merely undergone metasomatic change. It is found as one of the important 

 filling constituents of amygclaloids. It frequently develops at the contact 

 of two rocks, especially an igneous rock with other rocks, either igneous or 

 sedimentary. A list of different rock species which contain epidote includes 

 almost every variety of massive, schistose, semischistose, and little altered 

 igneous and sedimentary rocks. Epidote is, in fact, one of the most 

 important secondary constituents of all the silicates. It is an almost 

 constant accompaniment of the chlorites. Wherever the calcium-iron- 

 magnesium-silicate rocks break up, the magnesium passing into chlorite, a 

 part of the calcium and iron is likely to pass into epidote. The equations 

 for these alterations may be found under the minerals from which epidote 

 is derived. "Where epidote becomes so abundant as to be a chief constituent 

 it may give a name to a rock ; for instance, epidosite. From the foregoing- 

 statements it is apparent that epidote develops abundantly under mass- 

 static and under mass-mechanical conditions. It forms with ease and on a 

 great scale in the belt of cementation of the zone of katamorphism, and it 

 is probable that it develops to some extent in the zone of anamorphism. 

 Whether it forms at all in the belt of weathering can not be stated. 

 Epidote is derived from the following minerals: Anorthoclase, augite, 

 biotite, garnet, hornblende, melanite, microcline, orthoclase, the plagio- 

 clases, and the scapolites. 



Piedmontite, or manganese-epidote, is apt to replace epidote in those 

 schists and gneisses in which manganese happens to be an important con- 

 stituent. Thus it is rather common in certain manganese-bearing schists of 

 mon xlvii — 04 21 



