322 A TREATISE ON METAMORPHISM. 



Japan, in the manganese-chlorite-sericite-gneisses of eastern United States, 

 and at other localities. In some cases piedmontite occurs as nuclei sur- 

 rounded by ordinary epidote. Piedmontite is occasionally so abundant as 

 to be one of the chief constituents of rocks. 



Allanite occurs as an original subordinate constituent of a great number 

 of eruptive rocks, such as granite, rhyolite, diorite, tonalite, andesite, dacite, 

 and syenite. In short, it is a common accessory in the acid and intermediate 

 eruptives, but is not so characteristic of the basic eruptive rocks. It also 

 occurs in the metamorphic rocks, such as the schists and gneisses, especially 

 those which are calcareous, and it may occur also in the marbles. 



Alterations. — Definite alterations of zoisite, epidote, and piedmontite are 

 not recorded. But it is certain in the belt of weathering that zoisite and 

 epidote break up into calcite (rhombohedral ; sp. gr. 2.7135), quartz (rhom- 

 bohedral; sp. gr. 2.6535), iron oxides, kaolin (monoclinic; sp. gr. 2.615), 

 and perhaps gibbsite (monoclinic; sp. gr. 2.35); and piedmontite and 

 allanite alter into other minerals in a similar fashion. 



It has already been seen that in the alteration of mica, pyroxene, 

 amphibole, and other minerals chlorite and zoisite are frequent simultaneous 

 products which together use up all the material of the original minerals. 

 It has also been noted that the chlorite and epidote are abundantly 

 developed together in the sedimentary rocks. If the conditions so change 

 that these sedimentary rocks or other rocks in which epidote and zoisite 

 have formed in the zone of katamorphism become so deeply buried as to 

 pass into the zone of anamorphism, it is highly probable that the consti- 

 tuents which form epidote and zoisite and those which form chlorite reunite 

 to produce minerals that are on the average denser, such as mica, amphibole, 

 pyroxene, etc., out of which they are originally developed. This is believed 

 to be probable from the fact that in the most profoundly metamorphosed 

 sedimentary rocks, those which are true schists and gneisses, little or no 

 epidote and chlorite is contained, unless they have again been subjected to the 

 conditions of the upper physical-chemical zone. Such schists and gneisses, 

 having been derived from and traced into ordinary sediments, in all prob- 

 ability did originally contain both chlorite and epidote, which have doubt- 

 less united to reproduce heavy minerals similar to those from which epidote 

 and chlorite formed originally. 



