466 THE GEOLOGY OP MINNESOTA. 



[Basalt. Amygdaloid. 



to the feldspars, and imperfectly so in respect to the augite, yet there are inclusions 

 of it and other microlitic forms in the feldspars, which date probably from the last 

 consolidation of the magma. 



In general, this rock, and the last, are fresh, the minerals all being clear and 

 fresh, except for the slight effects of the reactions resulting from the hot waters and 

 gases that accompanied the intrusion and the congealation. 



One section. 



Age. Cabotian. N. H. w. 



No. 640. BASALT. 



At the water level under the hill formed by No. 639, about a mile east of Silver creek. Separated from 

 No. 039 by a red amygdaloid (as seen at Two Harbors*), eighteen or twenty feet thick. 

 Ref. Annual Report, x, page 64. 



Meg. Dense, fine-grained, nearly black rock. 



Mic. This rock resembles Nos. 632 and 635. In the section examined, with an 

 abundant matrix of devitrified glass, which is brownish and charged with fine opaque 

 matter (magnetite ?), are many microliths of feldspar. These are the only identifi- 

 able original minerals. No augite nor olivine can be seen. In some areas, however, 

 has been generated secondary quartz, which sometimes embraces poikilitically some 

 of the rock matter adjacent. Quartz does not generally permeate the rock, but the 

 undifferentiated magma serves throughout as a cement for the feldspars. 



One section. 



Age. Cabotian (?) 



Remark. It seems likely that this basalt is a thin lava sheet cotemporary with 

 those that alternate with amygdaloidal conglomerates, as noted, westward from 

 Little Marais. (Compare Nos. 629-632; also No. 117.) Still, it may be distinct from 

 them, and may prove to run below the Beaver Bay diabase, by reason of a fault. 



N. H. w. 

 No. 641. AMYGDALOID. 



Mouth of Knife river at the lake shore. 



Ref. Annual Report, ix, page 24; Annual Report, x, page 64. 



Meg. Like No. 91. Grayish green, much amygdaloidal, with thalite. 



Mic. The section examined is not much amygdaloidal, but exhibits the ophitic 

 structure on a grand scale. The augite is considerably altered, but extinguishes 

 sufficiently to show the size and orientation of the crystals, which embrace generally 

 a large number of feldspar microliths, comparable to the illustration by Irving in 

 plate IX, of his Monograph on the Copper-Bearing Rocks of Lake Superior. The 

 similarity is still further seen in the existence in both rocks of a large amount of 



Two Harbors, as here referred to, 18 not the town of Two Harbors, since established and grown into an important shipping 

 point of iron ore, but a small double bay, known by this name in 1877 and 1878, just west of Splitrock river. The name is here 

 preserved, as it has entered into the literature of the survey, while the locality of Two Harbors (since named) is designated Agate 

 bay, as long known and as referred to in the survey reports. 



