146 THE GEOLOGY OF MINNESOTA. 



[Dial. as.-. 



51 lies between them. They were probably all cooled at or near the surface, and 

 they show the volume of the basic and glassy floods which occurred at or near the 

 surface in Cabotian time in the vicinity of Duluth. N. H. w. 



No. 53. DIABASE (ivith olivi 



East Duluth. At the lake shore, N. W. % sec. 24, T. 50-14. Has a basaltic structure. 

 Ref. Annual Report, ix, p. 19; Annual Report, x, p. 139; Bulletin ii, pp. 10G, 107; American Association 

 for the Advancement of Science, vol. xxx, p. 162. 



Following is Wads worth's description: 



" This is microscopically a dark-gray, compact crystalline rock, showing lath- 

 sh;i[ed feldspars. Microscopically the section is seen to be composed of a brownish 

 tntyife. fi'/i/sjini; olivine, nmi/ncfifi', fijmlitt', and various secondary products. The 

 augite shows the ophitic structure first described by M. Michel Levy,* and later by 

 professor Pumpelly under the name ' lustre-mottlings.'f Attention has further been 

 called to this structure by professors A. Geikie,J R. D. Irving and J. W. Judd. 

 This structure consists of a large irregular area or various areas all belonging to the 

 same augite or diallage individual and cut by lath-shaped divergent feldspars. In 

 one form or another this structure is very common in the diabases, and usually is 

 the form of crystallization standing next to the granitic, in its coarseness of texture, 

 or one step nearer the fine-grained basalts. Many of the preceding described rocks 

 show the ophitic structure more or less perfectly, but not so well as this section. 

 The olivine is altered for the most part to greenish, yellowish brown, brownish 

 yellow and black serpentine, containing secondary magnetite sometimes marking the 

 former olivine fissures. Considerable dirty green viridite and secondary apatite 

 occur in the section, while some secondary biotite was observed in the vicinity of the 

 magnetite. The augite in places has the secondary cleavage of diallage." 



Afjr. Probably one of the Cabotian eruptives of the Taconic. See rain/ >/.:< 

 under No. 52, etc. N. H. w. 



No. 53A. DIABASE (u-ith olwine). 



Duluth. Same locality as No. 53. 

 Ref. Annual Report, ix, page 19. 



Meg. A decayed, earthy condition of No. -VI. 



No section. 



.\ijt-. Cabotian. u. s. <;. 



* Bulletin Socilti Oeoloyii/m' </< France, 1877 (3), vol. vi, p. 13(3. 

 i I'rnri-i-diniis Aiiii-i-irtni Aaademy of Artt and Scisncet, 1878. vol. xiii, p. 200. 

 J'rn nxii -liiins Royal Society, Edinburgh, 1880, vol. xxix, p. 495. 

 tt The C'lii/icr-llmrini/ Rocks, 1883, p. 42. 

 l| Quartert.il Journal, Gmlnny Society, 18S5, pp. 360, 361; 188fl, p. 68. 



