PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 787 



Greenstone.] 



eastwardly somewhat in the manner of a dike for a quarter of a mile, and can be 

 seen along the northern slope of the hill. (Compare No. 2095.) 



Mic. In a fine interlocking groundmass of feldspar and (apparently) of quartz 

 are phenocrysts of altered hornblende and of feldspar, the former evidently derived 

 from some augite (as they have frequently the forms of augite) and remnants of a 

 more highly refractive mineral. Throughout the whole are many sericite scales. One 

 section. 



Age. Archean (dike?). 



Remark. Except that the feldspars are not much twinned, this rock resembles 

 the esterellyte of Kekequabic lake, and it apparently occurs in much the same 

 manner. It is, unfortunately, as seen in the slide examined, much altered by 

 weathering. N. H. w. 



No. 1790. GREENSTONE. 



North from the Chandler mine, Ely. 

 Ref. Annual Report, xxii, page 5. 



Meg. Appearing rather massive, but pitted with depressions, like amygdules, 

 filled with a dark-green mineral. 



Mic. The supposed amygdule cavities are altered pyroxenes. They are not 

 wholly changed, but retain enough of their original substance to show their orien- 

 tation, which is negative, according to Lacroix's distinction (Mineralogie de France, 

 vol. I, page 568), i. e., the axis n f is nearer the vertical axis than is n g , in that respect 

 being like the augite of the porphyry at Kekequabic lake, approaching (egyrine. 

 These conspicuous phenocrysts are largely altered to hornblende and to chlorite, and 

 hornblende spicules and shreds are disseminated through the whole rock, giving it 

 the green color. The feldspathic element is fine, mostly microlitic albite twins, but 

 also interlocking-granular. There is no certainty of any old feldspars absorbed by 

 micro-granulitic encroachment, as the groundmass is evenly constructed of the feld- 

 spars already mentioned and the fine hornblendes. There is, however, much calrih- 

 showing a disengagement of lime. One section. 



Age. Archean (Keewatin). 



Remark. There is a narrow toothed border of hornblende, as seen in many 

 other instances, projecting beyond the partially altered pyroxenes, while the general 

 form of the pyroxene remains. It suggests that the so-called zoned hornblendes as 

 of the porphyry at Kekequabic lake, are not truly zoned because of secondary 

 growths, but that the difference of polarization colors, and all the contrasting 

 characters are due probably to the difference of surroundings during the single 

 development of a hornblende crystal, viz., that the darker, usually central portion 

 is the space of the original augite, but that beyond the bounds of the original augite 



