642 THE GEOLOGY OF MINNESOTA. 



[Amphibolyte. Tuff. 



These hornblendes are distinctly of two conditions of growth, like those seen in the 

 dike on Stuntz island (No. 872), and in a similar manner they polarize in two colors. 

 The central portion of these hornblendes is dichroic and sometimes still consists of 

 auffite, but the transparent second growths do not perceptibly show this quality (see 

 figure 37). The non-dichroic growths are fibrous and spreading at the ends of the 

 longitudinal sections, and also are scattered as slender threads through the matrix, 

 the fine, thread-like sections swimming off separately in the fine matrix, but without 

 much divergence. There are, besides, indistinct, non-polarizing, yellowish and dirty 

 spots which appear to be the residue of some original crystal forms which cannot 

 now be determined. One section. 



Age. Archean. 



Remark. This rock is very peculiar, and can only be said to be one of the prod- 

 ucts of the contact of the gabbro of the immediate vicinity on the elastics. N. H. w. 



No. 1048. AMPHIBOLYTE. (Tuff, modified.) 



Same place as No. 1047. Occurs as rounded and subangular masses in No. 1047, into which it apparently 

 shades. 



Ref. Annual Report, xv, pages 364, 395. 



Meg. Fine grained, dark gray. 



Mic. The rock (consisting almost entirely of hornblende) is essentially the 

 same as No. 1047, but much finer grained. The minute spicules and grains of horn- 

 blende appear between crossed nicols, much like muscovite scales, but when one is 

 cut favorably it can be seen to be of hornblende, with the same secondary enlarge- 

 ments and without parallel extinction. One section. 



Age. Archean (Keewatin). 



Remark. The hornblendic element here also must have been derived from a 

 fine debris of augite.* N. n. w. 



No. 1049. TUFF. (Modified.) 



East side of sec. 4, T. 64-7. Six feet above the water at the shore of Kekequabic lake. 

 Ref. Annual Report, xv, pages 364, 365, 395; Annual Report, xvii, pages 196, 205. 



Meg. Apparently a biotite gabbro, as judged in the field, but somewhat pebbly. 



Mic. This rock is porphyritic with hornblende in the same manner as No. 

 1047, but the secondary growths are not so noticeable, but are visible in sections 

 sufficiently thin. There are in the matrix with the hornblende crystals, some old 

 triclinic feldspars in small grains and fragments, which have the altered aspect in 

 the main of the feldspars described in No. 1044. The actinolite spicules penetrate 

 only their margins. Two sections. 



Age. Archean. 



Remark. This rock seems in its matrix to be allied to No. 1044, and in its horn- 

 blendic porphyroidal aspect to the hornblendic porphyry of Mallmann's peak, No. 751, 



* The sample preserved is apparently not exactly the same as that from which the slide was made. 



