PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 289 



Gabbro. Quartzyte.] 



edge, into a greenish hornblende which is the second step in the diallage alteration. 

 Apatite and magnetite are common and some biotite was observed. 



"Figure 1, plate IX, indicates the graphic oreozoon stage in the alteration of 

 this rock, while figure 2 of the same plate shows a more highly altered or a biotite- 

 hornblende-granite -form. The quartz contains microliths and fluid cavities." 



Remark. The foregoing is substantially the same that the writer would give as 

 a description of the appearance and composition of this rock. He would, how- 

 ever, call attention to the unusual conditions which surround this rock in its struc- 

 tural relations. It is evident that this altered condition is due to the contact with 

 quartzytes of the region and not to any normal or widespread change which could 

 give rise to such a rock. The plagioclase therefore is more likely to be original than 

 secondary. The alteration which pervades the reddened areas, permeated with 

 micropegniatitic quartz, seems more likely to be the residue of the quartzyte inclu- 

 sions which did not respond to the crystallization which affected the quartz. As the 

 quartz of the pegmatyte darkens over considerable areas simultaneously, it is plain 

 that one orientation prevailed throughout areas which were not actually in contact, 

 but were separated by a " feldspathic " ingredient which did not crystallize. Whether 

 that ingredient was an original feldspar, which now has lost its power to crystallize, 

 as believed by Dr. Wadsworth, or was simply the rejected impurities left after the 

 quartz was regenerated, is a question worthy of consideration. That the circumstances 

 of injection of a molten basic rock between the layers of an acid one would be favor- 

 able for the absorption of silica throughout the region of contact by the basic rock, 

 and for the rearrangement of crystalline orientation of those portions of the quartzyte 

 included in the zone of greatest heat, cannot be questioned. Heated alkaline waters 

 would be generated. These would seize on the quartz and would transform it readily 

 into such shapes as the adjacent surroundings permitted. Several other members 

 illustrate the same transformations. The viridite is probably penninite. It has a 

 distinct cleavage and a hexagonal form. In a new section the plagioclase is seen to 

 be posterior to both augite and olivine. Three sections examined. 



The field description, published in the Ninth Annual Report, page 63, makes it 

 plain that this rock is an intrusive in the Animikie along the bedding, being the 

 first published observation of an actual intrusion in that manner in the Animikie. 

 The intruding rock here is evidently a spur from the great dikes of the region, and 

 is of Cabotian age. N. H. w. 



No. 264. QUARTZYTE. (Altered.) 



From the shore of Wauswaugoning bay due north (by compass) from Birch island: near the centre of E. J 

 sec. 30, T. 64-6 E. Sample shows changed quartzyte enclosed in the gabbro (No. 263). 



Ref. Annual Report, ix, page 63; Annual Report, x, page 141; Proceedings American Association for the 

 Advancement of Science, vol. xxx, page 165. 

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