PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 87 



Gabbro. Granite. ] 



menaccanite. The meager percentage of magnesia corresponds to the low content 

 of hornblende and diallage in the rock. The apatite in the rock amounts to 0.81 

 per cent." 



Seven sections examined. 

 A<j<>. Cabotian of the Taconic (or Lower Cambrian). N. H. w. 



No. 1 A. GABBRO (luith orthoclase) . 



From the high land, Duluth. Same rock as No. 1, but further northeast; intersection of Fifth Avenue 

 East and Seventh Street; near a contact with No. IB. 



Ref. Same as for No. 1. Also, Annual Report, x, pages 41, 141; Proceedings of the American Association 

 for the Advancement of Science, vol. xxx, page 165. 



Mi'//. As No. 1 approaches the "red rock" of the region (No. IB) it begins to 

 acquire other minerals, and a reddish, feldspathic spottedness, as expressed by this 

 rock and by No. 5. At the contact the grain is sometimes considerably finer than 

 at a distance from the contact. The section examined seems to have been derived 

 from near the contact. It was described in the Proceedings of the American Asso- 

 ciation for the Advancement of Science, Cincinnati meeting (1881), when it was said 

 to contain " labradorite, uralitic augite and titaniferous magnetite. Some of the 

 augite is changed toward viridite; orthoclase in occasional grains; " and it was placed 

 amongst the " mixed igneous and sedimentary rocks." It principally differs from 

 No. 1 in having a visibly greater proportion of the orthoclase element. It is not an 

 independent rock, but an altered phase of No. 1, as will appear more fully in the 

 descriptions of some of the other samples. 



One section examined. 



Age. Cabotian. N. H. w. 



No. IB. GRANITE. (Red.) 



Duluth; probably N. W. 14 sec. 34, T. 50-14. 



Ref. Annual Report, ix, pages 11, 12, 17, 18; Annual Report, x, pages 41, 140, 201, 204; Annual Report, xiii, 

 pages 100, 103; Bulletin viii, pages xxx, xxxiii; American Association for the Advancement of Science, vol. xxx, 

 page 163, 1882. 



Compare R. D. IRVING, Mon. v, U. S. Geol. Survey, page 119, 1883. 



There are two hand samples of this number, both of which have been analyzed. As the two differ con- 

 siderably, they will be described separately as Nos. IB and IB '. 



Meg. A medium or rather fine-grained granitic rock of a brick red color. Com- 

 posed principally of two minerals: a reddish feldspar and a black or greenish black 

 substance, apparently hornblende; the former is in greater amount than the latter. 

 Quartz, in smaller grains than the other two minerals, is not very evident, but is 

 scattered throughout the specimen. A.few small yellowish areas are also seen. 



Mir. The section shows that the rock is highly altered, but as it is typical of 

 certain phases of the " red rock " of the Cabotian, to which Irving has applied the 

 term augite syenyte, it will be described rather fully. The structure is granitic, 

 although the feldspar has a tendency to an idiomorphic development, and the following 



