284 A TREATISE ON METAMORPHISM. 



They also occur in the metamorphosed calcareous fragmental sediments. 

 Where iron is not abundant, as in the marbles, tremolite is the mineral 

 which ordinarily develops. Where ferrous iron is plentiful actinolite 

 normally forms. Where iron is the chief or only carbonate, griinerite 

 ordinarily develops. Tremolite and actinolite also occur as alteration 

 products in igneous rocks, being noted in diabases, gabbros, and more basic 

 rocks. The secondary products frequently take the form of asbestos and 

 jade. They are frequently associated with talc and serpentine in steatite- 

 schists or serpentine-schists. Often, also, tremolite and actinolite are asso- 

 ciated with pyroxene, epidote, and chlorite. These amphiboles also occur 

 in veins. Summarizing, as metamorphic minerals, tremolite is derived from 

 diopside, dolomite, and olivine; actinolite from ankerite, bronzite, hyper- 

 sthene, olivine, parankerite, and sahlite. 



Cummingtonite, the monoclinic amphibole corresponding in composi- 

 tion with the orthorhombic amphibole anthophvllite, occurs in various 

 schists of metamorphic origin. It is not known as an original constituent 

 of the igneous rocks. 



Griinerite occurs most extensively in connection with magnetite and 

 quartz, or with quartz alone, thus constituting grunerite-magnetite-quartz- 

 schists, or grunerite-quartz-schists. The griinerite in such cases often 

 develops as a secondary product from the alteration of siderite, as explained 

 on page 245. Greenalite, probably having the formula FeSi0 3 .nH,0, occurs 

 extensively, as in the Biwabik formation of the Mesabi series of Minnesota." 

 If such material were so deeply buried as to be altered under the conditions 

 of the zone of anamorphism, dehydration would take place and griinerite 

 would be formed. The mineral also occurs in the garnetiferous micaceous 

 schists; but in some of these rocks the griinerite itself develops from the 

 siderite, as in the case of the pure griinerite-quartz-schists and griinerite- 

 magnetite-quartz-schists. 



Hornblende is the most abundant of the amphiboles, and has a very 

 widespread occurrence, being found as a principal constituent in various 

 igneous rocks, including plutonic and volcanic rocks, and among the latter 

 both in lavas and in tuffs. It also is a constituent of some of the sedi- 

 mentary rocks. It is a chief constituent of many of the metamorphosed 



"Leith, C. K., The Mesabi iron-bearing district of Minnesota: Mon. U. S. Geol. Survey, vol. 43, 

 1903, pp. 101-115. 



