MINERALOGY AND PETROLOGY. 959 



"Original" hornblende. Cummingtonite.] 



No. 794aG. Hornblende lost by decay, either chloritized or magnetited. 



No. 794bG. Clastic rock, probably tuff; fragments angular, embracing some quartz; no hornblende. 



No. 795G. Distinct augite remnants. Hornblende is green. 



No. 79CG. Much augite remaining as grains in the hornblendes. 



No. 797G. Some hornblendes are apparently original, but some have central nuclei not of augite ; others 

 are wholly chloritized except at the centre, where are apparently augitic grouped grains, which grains appear to 

 be in part epidote. 



The central grains not augite (above) are largely leucoxene, but embrace, apparently, some feldspar and 

 and perhaps some chlorite, while the general hornblendic orientation runs through them, causing imperfect 

 extinction. 



It seems likely, therefore, from the occasional existence of augite remnants in 

 these hornblendes, that they are modified conditions of augite. There is no doubt 

 that this rock acts the part of an igneous intrusive, in the same manner as the granite 

 and massive porphyry of Kekequabic lake, and in consonance with that fact the 

 hornblendes appear almost entirely "original," in the usual petrographical sense. In 

 the broad sense, however, that has been indicated (page 940), they are in a secondary 

 rock, and are not original. They are to be considered original only in the sense that 

 quartz and orthoclase are original in granite. 



So far as the writer has observed, hornblende appears in the Archean only as a 

 result of alteration of augite or some other minerals under conditions of profound 

 metamorphism. 



The varying amounts of alumina and of iron present and available under the 

 conditions that promote such alteration seem to be the prime cause of the different 

 varieties of hornblende that have been noted. Brown hornblende is a frequent accom- 

 paniment of the gabbro rocks where they carry notable amounts of magnetite, and 

 especially where the gabbro is shading off to muscovadyte (Nos. 703, 1288, 1292, 1711). 

 Cummingtonite. This name is preferred, on the authority of Hintze,* who, on 

 account of chemical composition, has assigned that mineral, which in Michigan is 

 apparently the analogue of that found in Minnesota, to this species. The mineral 

 here referred to was named actinolite by the geologists of Michigan and Wisconsin, 

 and the rocks to which it gives character were for several years known as actinolite 

 schists, or actinolite-magnetite schists. Subsequently, after the more minute exam- 

 ination and analysis by Lane and Sharpless,f they were known for several years as 

 grunerite schists. This mineral is known microscopically, amongst the amphiboles, 

 for its multiple, narrow twinning, its high double refraction and its fresh, light- 

 colored appearance. It is common on the Mesabi Iron range in connection with the 

 iron-bearing member of the Animikie (Nos. 437, 766, 976, 1365), where the rocks 

 have been somewhat metamorphosed by proximity to the gabbro revolution. It has 

 also been seen several times in the highly metamorphic iron ores of the muscovadyte 

 zone (No. 397(a)H) and occasionally elsewhere (Nos. 1365, 1710). 



*Hanrto<ich dcr Mincraloi/ir, p. 1280. 



^American Journal of Science, vol. xlii, p. 505, 1891. 



