Washington — Study of the Glaucophane Schists. 59 



the composition of the original material, as necessary to the 

 formation of the mineral. 



Again we find glaucophane schists in the " Grundgebirge " 

 or Lower Cambrian at lie de Groix, Neocomian among the 

 Pacific Coast, Post Cretaceous in Attica, and probably of other 

 ages elsewhere. Similarly amphiboHtes are found of various 

 ages. 



Lastly, the fact that these rocks are not found generally dis- 

 tributed over the earth, as are the amphiboHtes, but occur in 

 well-defined zones or regions of metamorphic rocks, points to 

 the existence of some peculiarity in the conditions of the meta- 

 morphic processes involved. This occurrence in strongly 

 marked regions is exemplified by their presence in Greece and 

 the Archipelago, the Piedmont Region in Italy, Japan and 

 along the Pacific Coast of this country. 



Summary. — The glaucophane schists belong to two main 

 groups, sharply separated from each other. The larger one is 

 basic, composed chiefly of glaucophane and epidote, often with 

 abundant garnet, zoisite, diallage, and sometimes smaller 

 amounts of mica, feldspar and quartz, and rutile and titanite as 

 frequent accessories. Chemically these closely resemble the 

 composition of the rocks of the gabbro family, and are appar- 

 ently divisible into two subgroups, one high in CaO, the other 

 low in it. These are in most cases almost undoubtedly derived 

 from such igneous rocks or their tuffs, but also possibly in rarer 

 cases from sediments or slates of similar composition. 



These basic glaucophane schists scarcely differ in chemical 

 composition from the amphiboHtes and eclogites, and the dif- 

 ference in their formation is probably to be ascribed to differ- 

 ences in the conditions of metamorphism. 



A smaller, but widely spread, group is acid in composition, 

 and these are composed largely of quartz and glaucophane, 

 with mica and sometimes albite. These are derived from 

 cherts, quartzites or quartzose shales and sandstones. 



The existence is indicated of a third, still smaller, group of 

 intermediate mineralogical composition, and chemically like 

 the diorites. 



The glaucophane schists are apparently the result of both 

 regional and of contact metamorphism, and in many regions 

 they occur together. This last seems to be the rule in glauco- 

 phane schist areas of any size, and where only the one kind is 

 found the area is apt to be small. 



Locust, N. J., September, 1900. 



