PEtROGRAPHIC GEOLOGY AND DESCRIPTIONS. 678 



Crabbro. Diabase . ] 



some of the labradorites, cutting their borders, but being cut by others. Olivine is 

 quite plentiful. One section. 



Aye. Cabotian. N. H. w. 



No. 1137. GABBRO. 



Krom a point a short distance west of the meander stakes between sees. 24 and 25, T. 61-12, Birch lake. 

 Kef. Annual Report, xv, pages 332, 399 ; Annual Report, xvii, pages 198, 207. 



Meg. Fine-grained gabbro. 



Mii: The augite is dialltii/ic, and is also sometimes crowded with coarser inter- 

 lamellar impurities, which, as in the last, do not coincide with the direction of the 

 diallagic structure. Olivine is quite abundant. It was cotemporary with, or later 

 than, either feldspar or augite. Magnetite is accompanied by biutiti: The olivine is 

 sometimes changed peripherally to bowlingite( ? ) and in the immediate vicinity is 

 developed a finely radiated, rather highly doubly refracting lamello-flbrous mineral, 

 which, with positive elongation, colorlessness in common light, seems to be a form 

 of ti-1'ino/ifr. but this is closely mingled with a light greenish fibrous mineral which 

 has about the same -structure, arranged frequently along the borders of the tremolite 

 areas as if intimately related to the tremolite, and sometimes they intertwine or 

 mingle. This is pleochroic and is evidently the remains of a green hornblende 

 ( adinolite) from which the other is derived. One section. 



Age. Cabotian. N. H. w. 



[NOTE. Nos. 1141 to 1263, inclusive, were collected outside the state of Minne- 

 sota and are not here described, excepting Nos. 1178, 1200 and 1203, which are 

 described below. The field relations and preliminary designations of Nos. 1141 to 

 1263 are given in the Sixteenth Annual Report, pages 13-60, 114-119.] 



No. 1173. DIABASE. 



Thessalon point, Canada. Original Huronian area. 

 Ref. Annual Report, xvi, page 18. 



Mey. Schistose and slaty diabasic rock. Logan's "green chlorite slate." 



Mic. The rock is composed of microlitic feldspars, lying in a matrix of altera- 

 tion products, chief among which is ncfittolite, but in which also is leucoxene, zoisite 

 and apparently epidote. One section. 



Age. Keweenawan ( ? ) 



Remark: This is a part of the great greenstone or diabase seen a few miles 

 further north and west, which is there later in date than the great basal quartzyte 

 on which it lies, the quartzyte being considered of the age of the Potsdam at Pots- 

 dam, New York. N. H. w. 



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