PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 881 



Porphyry. Graywacke.] 



No. 2268. PORPHYRY (or porphyrel). 



On the same portage, near the same place as the last. 

 Ref. Annual Report, xxiv, page 8] . 



Mcy. A gray rock with coarse, porphyritic white feldspars. 



Mic. This is a compacted debris, partially regenerated by new minerals, but 

 showing still plainly the decayed original stuff. There is a little fine quartz, but the 

 rock is almost wholly feldspathic. Epidote and a little hornblende (often chloritized) 

 give it a darker color. The twinned and striated porphyritic feldspars contain 

 epidote, Muscovite, calcite as products of alteration, also occasionally a hornblende 

 spicule. These secondary minerals are distributed evenly throughout the feldspar 

 "phenocrysts," i. e., they do not affect the central areas. In that respect they differ 

 from the secondary products of old feldspars that have been regenerated, as in 

 granites. The only rule that can be observed is that epidote is apt to be more 

 abundant in the peripheral parts. Most of the feldspar grains are so fine that they 

 have been destroyed and blended in the general matrix by a micro-granulitization, 

 and they can then be detected only by the contrast of their fineness with the coarser 

 grain of the surrounding parts. The abundant calcite visible in some parts of the 

 slide does not probably arise wholly from alteration of the phenocrysts, since some- 

 times such phenocrysts cut off the calcites sharply, with a distinct margin which 

 encloses none of them. They seem to be due to some calcareous clastic ingredient 

 which has been lost by alteration. 



In the slide is one small crystal of sphene, also several irregular pyrites. One 

 section. 



Age. Intrusive in Upper Keewatin. 



h'einark. This rock stands intermediate, in point of recrystallization, between 

 the porphyry of Kekequabic lake and the porphyrel of Zeta lake. N. H. w. 



No. 2269. GRAYWACKE. (Hardened, feldspathic. ) 



On the same portage, near the same place as the last. 

 Ref. Annual Report, xxiv, page 81. 



Meg. A fine-grained, light-green-gray rock, having the appearance of an igneous 

 intrusion, sharply much jointed. 



Mic. In the field it was suggested that this rock might be due to the hardening 

 of a siliceous mud, but it was, instead, a fine feldspathic mud with very little quartz. 

 The section presents distinctly a field that is made up largely of small, angular frag- 

 ments of plaaioclase, very much altered with development of Muscovite, epidote and 

 calcite, being a rock essentially the same as the last, and differing in the absence of 

 phenocrysts. One section. 



Aye. Upper Keewatin. N. H. w. 



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