584 THE GEOLOGY OF MINNESOTA. 



[Amphibolyte. Oamptonyte. 



the older crystal in such a manner as no crystal can be supposed to have been built 

 up without interruption. These colorless parts are certainly not due to dynamic 

 degradation. They are not broken. They are the purest and the highest doubly- 

 refracting parts. 



They are, no more, due to the decoloration of a once wholly idiomorphic crystal- 

 lization, for their present integrity of form and of optic characters is inconsistent 

 with such transformation. They extinguish simultaneously with the colored areas, 

 but the difference of double refraction indicates that these increments may be of a 

 different species of hornblende. 



Zoisite is quite abundant in this rock. 



Six sections. 



Age. Archean dike in Keewatin. 



!!< mark. Except for the double development of the hornblende (which may 

 not be a fatal objection), this rock might be classed as ca>nptoni/te, a rock that is found 

 in form of dikes in the Adirondack region and in Vermont. A similar dike occurs 

 at Ely. (See No. 1786.) On the nature of these "secondary growths" see the 

 remark after No. 1786, at Ely. N. H. w. 



No. 872A. AMPHIBOLYTE (?) 



" No. 872A was taken from the side of a narrow dike of the main system on Stuntz island [Vermilion lake]. 

 This dike is not more than one and a half inches thick where the piece was taken off, and pinches out entirely 

 toward the west further, in about ten feet." 



Ref. Annual Report, xv, page 310. 



Meg. A fine-grained, greenish-gray, rusty-weathering rock. 



Mic. The section is composed largely of a fibrous, colorless to greenish mineral, 

 which appears to be a form of hornblende. Besides this mineral there is a little 

 quartz, perhaps also some feldspar, and considerable zoisite. The latter occurs in 

 irregular grains throughout the section and is also collected in areas composed of 

 larger grains. The section is clouded by a light yellowish material; when examined 

 with a high power this yellowish material is seen to be composed of minute grains 

 having a rather high index of refraction and little effect on polarized light; they 

 may also be zoisite. The section shows one, considerably altered, porphyritic feldspar. 



One section. 



Age. Dike cutting Keewatin rocks. 



Remark. The original nature of this dike rock is uncertain. It is now much 

 altered. It is quite likely that it is of the same nature as No. 872, which is suggested 

 to be a camptonyte. u. s. a. 



No. 873. CAMPTONYTE (?) 



Prom another set of dikes on Stuntz island, Vermilion lake; sec. 21, T. 62-15 W. 

 Ref. Annual Report, xv, pages 308, 314, 387. 



