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MUSEUM OF COMPAUATIVE ZOOLOGY. 



119 



and rounded, and penetrated bj the base the same as they are in mod- 

 ern rhyolitcs ; and in like manner they are seen to be of prior origin to 

 the consolidation of the lava. The quartz here, as in the rhyolites, plays 

 the same part that oli\^iuc does in, the basalts and hornblende in the 

 andesites. It is a foreign ingredient, of prior origin to the lava, and 

 has not crystallized out of it.* The quartz contains stone, vapor, and 

 fluid inclusions. The majority of the fluid and vapor (empty) cavities 

 are arranged along fissures, as if they had been formed by hot waters 

 depositing silica in fissures. The feldspar is thoroughly kaolinizcd, 

 and, like the quartz, is frequently in rounded and broken pieces, 

 which show that it was formed prior to the consolidation of 'the lava. 

 Considerable secondary quartz occurs in the groundmass, but the base 

 affects polarized light but little, if at all. 



It is intended to figure this section in connection with some of the 

 rhyolites of the Cordilleras, for if ever a section showed conclusive 

 proof that the view, that volcanic action did not commence until the 

 Tertiary epoch, is fallacious, this is one. No. 553, from the same locality, 

 has a similar, but lighter brown and less abundant groundmass. This 

 holds numerous yellowish-red and clay-colored feldspars, as well as much 

 quartz. This rock, like the preceding, is an old rhyolite (quartz por- 

 phyry). The base is an olive-brown, fclty mass, holding opacito grains, 

 quartz, feldspar, and decomposed black hornblende. Tiie characters of 

 the quartz and feldspar are like those in the preceding rock. On the 

 edges and in the cleavage planes of the feldspar, copper has been depos- 

 ited in both of these rocks. No. 554, from the same locality, is a grayish- 

 red, granitoid rock. In the thin section this is seen to be like the more 

 granitoid trachyte pebbles found towards Torch Lake, and described 

 before. This contains much secondary quartz, and a little that appears 

 to bo primary. The feldspars are decomposed, and the quartz is so 

 arranged in them that they show the graphic characters. The rock 

 holds numerous rows of ferrite globules, which rows are arranged in a 

 radiate form, giving to the slide a spherulitic appearance. A little of 

 the feldspar is seen to be triclinic. No. 5G2 is a pretty brown rhyolite 

 (felsite) with a very compact groundmass, holding some minute feld- 

 spars, 



feldspars are seen to be largely triclinic, while the groundmass is a 

 brownish devitrified aggregate of secondary quartz, feldspar, and ferrite. 

 The finer portions of the Calumet conglomerate arc seen microscopi- 

 cally to be composed of the rhyolitic and trachytic detritus, the former 



* Bull. Mua. Comp. Zoology, V. 277-282. 



This was taken from the conglomerate of the Osceola mine. The 



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