PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 719 



Greenwacke. Dioryte.] 



Meg. Hornblendic schist. 



Mic. The rock consists of honthlcnde in crystals and in fragments, lying in a 

 fine matrix 'of granular interlocking quartz and feldspar, through which sometimes 

 the slenderest needles of hornblende run without deviation, and the larger horn- 

 blendes have been slightly enlarged by borders and by spicules from the extremities 

 of the sections. They have been originally twinned. There are no large feldspars 

 visible in the section. One section. 



Age. Archean (Keewatin). 



Hcniiirl,-. The hornblendes in this rock frequently show contrasting colors, /. <'., 

 the central portions, with outlines of former augite crystals, are darker colored. 

 This character, seen in many instances, is shown in another place to have been due 

 to the manner of origin of the hornblendes, and indicates that this hornblendic 

 element was derived wholly from alteration of augite. The original rock was 

 probably a volcanic tuff. Compare No. 1060. N. H. w. 



No. 1410. DIORYTE. (Schist with quart*.} 



From the top of the same island. 



Ref. Annual Report, xvi, pages 102, 105, 125. 



Meg. The same rock mass. 



Mic. This rock has a surprisingly different aspect. It is much coarser grained. 

 Tim /lori/li/riidm, though ragged and fragmentary, are large and are accompanied by 

 hiofite and epidote, the former encasing the latter poikilitically (compare No. 1106). 

 In addition, there are conspicuous triclinic feldspars twinned on the Carlsbad and 

 albite types, as well as on the pericline. They appear to be the same " old " feldspars 

 as characterize the granite and the porphyry of the region. They are sometimes 

 interfered with by the hornblendes and enclose small hornblendes as if the horn- 

 blendes, or that from which the hornblendes are derived, were of earlier date. Along 

 with this are areas which appear to be of pebbly shapes and sizes, consisting 

 of micro-granulitic quartz and feldspar, and lastly there are coarsely granitic 

 interlocking quartzes, which appear to be the most recent element of the rock. 

 Add to this a very little nt/cife and miit/m'titi' and hematite and the description is 

 complete. 



Age. Archean (l ! pper Keewatin). 



Ht'tm t r/.-. This rock does not differ essentially from the "granite" of the south 

 sideof Kekequabic lake (compare No. 1414). The hornblende has a distinct tendency 

 toward a green-blue color when cut perpendicular to one of the horizontal axes. It 

 has M e nearly parallel with the elongation, and hence is not riebeckite. The bight 

 of the island is about twenty feet. See the field description, Sixteenth Annual 

 Report, page 103. 



