780 THE GEOLOGY OP MINNESOTA. 



[Porphy rel . 



mass; the other is destitute of such crystals, very fine grained, and is a part of the 

 schist or greenish conglomerate of the region. The feldspars in the latter have 

 been eaten up by replacement by a micro-granulitic encroachment, leaving only 

 remnants of the original crystals. Honil>I<-ntle is in small crystals (after augite) and 

 as irregular shreds, running down to very fine particles. As a secondary product it 

 appears that actinolite needles have been developed. These are not connected with 

 the hornblendes as borders or secondary growths, but are scattered through the fine- 

 grained matrix. Several grains of spJirne are evident in this slide. Two sections. 



Age. Archean (Keewatin). 



Remark. The relation of this rock to the porphyry of the knob is problematical, 

 /. c., whether it is anterior or posterior to the porphyry. The feldspars have the 

 same much twinned appearance, so far as they are preserved. If the field obser- 

 vation is correct that there is a gradual passage from this to the porphyry, there is 

 a genetic relationship which, while in accord with the field observations at the time 

 they were made, seems also to be in accord with the microscopic features. The only 

 difference between this rock and the porphyry prevalent on the knob is the different 

 conditions of the feldspar crystals. Compare No. 1770. N. H. w. 



No. 1769. PORPHYREL. (Cam/loiix'nitic. .) 



From the north side of the narrows of Zeta lake. 

 Ref. Annual Report, xxi, page 159. 



Meg. Conglomeratic. 



Mic. The slide does not vary in a manner characteristic of a conglomerate, but 

 it is more like a volcanic tuff. It consists of the peculiar feldspars and of uralite from 

 legyrine like the porphyry of Kekequabic lake, including some sphene and mnyiti'lift'. 

 These are generally entirely distinct, but occasionally a grain of aegyrine is wholly 

 enveloped in a feldspar. There are also round small feldspars within the cegyrine. 

 The surrounding matrix is composed of the same materials, but in a finer state of 

 comminution. There is here visible none of the secondary micro-granulitic encroach- 

 ment on the feldspars. The hornblende is frequently conspicuously zoned, but does 

 not show the fibrous additions seen in some of the schists at Kekequabic lake. One 

 section. 



Age. Archean (Upper Keewatin). 



Remark. The name porphyrel was applied to this rock in the prosecution of 

 the field examination by Dr. A. Winchell. It is a rock like the Kekequabic Lake 

 porphyry, but here plainly and abundantly conglomeratic. N. H. w. 



No. 1770. PORPHYREL. (Conglomeratic.) 



South side of the narrows of Zeta lake. 

 Ref. Annual Report, xxi, page 159. 



Meg. Conglomeratic. 



