3H SMITHSONIAN MISCELLANEOUS COLLECTIONS IvOL. 45 



Origin of Unakitc 



The two rocks have originated from the same magma, and the 

 constituents necessary to produce the epidote are present in the hy- 

 persthene-akerite. These are the pyroxenes and plagioclase. In 

 some sections, as already noted, under hypersthene-akerite, horn- 

 blende is present. Its very limited occurrence, however, precludes 

 the possibility of its having been the chief source of the iron of the 

 epidote. It is a well-known fact that epidote is a common mineral in 

 regions which have undergone dynamic metamorphism,^ and such 

 influence may have had its effect in the present instance, for the wavy 

 extinction and bent albite lamellae (pi. lxxi, b) indicate that the 

 region has been subjected to some stress. The relatively large con- 

 tent of water in the akerite is suggestive. But the chief cause of 

 epidotization is perhaps due to the action of percolating meteoric 

 waters (hydrometamorphism). The change of the pyroxenes to 

 epidote may be seen in many of the sections studied. It begins along 

 the cleavage cracks and on the edges of the minerals, gradually re- 

 placing them, thence extending to the plagioclase and ultimately 

 replacing this mineral and orthoclase (pi. lxxi, a). It is accom- 

 panied by a separation of iron oxide in skeleton forms which gradu- 

 ally coalesce, producing the massive fractured forms noted under 

 unakite. It was thought that this fractured phase of the iron oxide 

 might be due to a partial removal of its mass to form epidote, but 

 the resistance of this mineral to alteration precludes this possibility. 

 The presence of much potash does not militate against this origin by 

 percolating waters, for the potash varieties of feldspar, as is well 

 known, are far more resistant than the soda-lime varieties.^ 



Magnesium is also frequently removed in greater proportion than 

 lime.^ In this connection, the presence even of small amounts of 

 soda and magnesia is suggestive, for it indicates the former 

 presence of those minerals involved in the formation of the epidote. 



Epidosite 

 The term epidosite has been used in the previous pages for a 

 quartz-epidote combination — the non-feldspathic phase of the una- 



^ Lindgren, Metasomatic Processes in Fissure Veins, Trans. A. I. il#. E., 

 xxx, 611. 



^ " Among the feldspars, the potash varieties are, as a rule, more refractory 

 than the soda-lime or plagioclase varieties. This is shown not merely by our 

 own observations, but by those of others as well. Roth shows from analyses 

 of fresh and weathered phonolite, nepheline-basalt, and dolerites that the loss 

 of soda is almost invariably greater than that of potash." — Merrill, Rocks, 

 Rock-zveathering and Soil, p. 236. 



" Ibid., p. 239. 



