PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 149 



Aporhyolyte. Diabase. Vein material.] 



No. 56. APORHYOLYTE (?) 



Duluth. A condition of No. 55. 

 Ref. Annual Report, ix, page 19. 



Mcy. Almost exactly like No. 55A, except that the porphyritic red feldspars 

 are more abundant. 



Mic. This rock is similar in general aspect to Nos. 50 and 55A, but the back- 

 ground of the rock, instead of being composed of quartz in minute grains, is made up 

 of larger areas of quartz holding the other materials poikilitically. The slide contains 

 one small black mass which is composed of black granular material (magnetite), in 

 which are a few feldspar microliths and one highly altered feldspar phenocryst. 



One section. 



Age. Cabotian. u. s. G. 



No. 57. DIABASE. 



East Duluth. From a dike cutting No. 56, which is said to be a condition of No. 55, running N. 5 E., four 

 feet wide. 



Ref. Annual Report, ix, page 19; Final Report, i, pages 196-199. 



Megascopically this is a brownish, dark, compact and fine-grained rock. 



Microscopically it is an ordinary diabase, consisting of lath-shaped plagiodases 

 and pyroxenic element much obscured by change. Magnetite is abundant and hema- 

 tite rare. 



One section. 



Chemical analysis. The following analysis was made by Prof. J. A. Dodge and first published in the 

 Final Report, vol. i, pages 198, 199: 



SiO 2 48.51 



A1 2 O 3 13.79 



FeO / 



CaO 8.34 



MgO 4.81 



K 2 O 0.19 



1.67 



Total 96.65 



The specific gravity was found to be 2.95 by one determination, and 3.005 by another. 



Age. Manitou. N. H. w. 



No. 57A. VEIN MATERIAL. 



Duluth. Just east of No. 57, from another dike. 

 Ref. Annual Report, ix, p. 19. 



Meg. A roughly laminated mass of quartz, calcite and a dark green chloritic 

 material. Evidently from a vein. 



Mic. The sections show aggregates of calcite, chlorite, hornblende (?) and 

 finely crystallized quartz. A considerable amount of ilmenite, largely altered to 

 leucoxene, is also present. 



Three sections. 



Age. Vein material in a Manitou dike. u. s. G. 



