PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 101 



Diabase.] 



No. 6A. DIABASE (u-lth olivine). 



Duluth. An extensive outcrop formerly existed beside the railroad. This is probably an equivalent of 

 No. 43, and a phase of No. 6. 



Ref. Annual Report, ix, pages 12. 17. 



Meg. A dense, rather fine-grained rock, dark gray in color, having the aspect 

 of an ordinary diabase. 



Mic. The/rA/.s/^/r of the first generation is evident but in fragmentary crystals. 

 Apparently the same feldspar has taken part in the second consolidation. At least 

 the distinction between the first and second crystallizations is so obscure that appar- 

 ently the same species grades from one to the other. They are decayed and much 

 twinned. 



The nlir'nii' is changed to an almost isotropic substance, with a light green tinge; 

 along the cleavages of the original olivine much magnetite is gathered. 



Any tit-, much altered, is greenish, but shows its ophitic relations. 



(JiKirtz is sparse, likewise /ti/r/fi'. M<inc1i(e is not abundant. 



Two sections examined. 



Age. Probably Cabotian of the Taconic. N. H. w. 



No. 6B. DIABASE. 



Duluth. In immediate contact with No. 6A. Undistinguishable from No. 6A, of which it is probably a 

 portion, but more plainly porphyritie, in thin section, with feldspar. 

 Ref. Annual Report, ix, page 12. 



One section examined. 



Age. Probably of the surface Cabotian eruptives. N. H. w. 



No. 60. DIABASE. (Spotted with red, and porphyritie.) 



Duluth. Foot of Lake street. Essentially the same rock as Nos. 6A and 6B. It is, however, spotted with 

 irregular small areas which apparently consist largely of orthoclase and pyrite. The red color faintly pervades 

 the entire specimen. Microchemical test for potassium made on the red substance gave large cubes of fluosili- 

 cate of potassium; also, many crystals indicating lime and soda. The whole rock is finely porphyritie with a 

 plagioclase. The red spottedness should not be mistaken for an amygdaloidal structure. The red areas are of 

 another rock and are foreign inclusions in the basic eruptive. 



lii'f. Annual Report, ix, pages 12, 18. 



Age. Probably Cabotian. N. H. w. 



No. 7. DIABASE (?) 



Duluth. Between Second and Third avenues, close to the water. Underlies immediately No. 7A. Dip, 

 E. 18. 



Compare No. 42. 



Ref. Annual Report, ix, pages 12, 18; Annual Report, xiii, pages 100, 102; Annual Report, x, pages G3, 109, 

 140; Bulletin viii, pages xxx, xxxiii; American Association for the Advancement of Science, vol. xxx, page 163. 



Meg. A dull, brownish, compact rock of fine grain. Composed of small crystals 

 of reddish feldspar in a darker, very fine-grained groundmass. A very few porphy- 

 ritie red feldspars are present; these are not more than five millimeters in length. 

 One of them is a simple twin. There are two cavities in the specimen which are 



