532 THE GEOLOGY OF MINNESOTA. 



[Dioryte. Diabase. 



Mic. The section is that of an altered diabase. There are numerous lath-shaped 

 , usually much altered, in a confused background composed of green 

 ', fltloritc and iaynftitc. 

 One section examined. 



Aye. Keewatin. u. s. . 



No. 750. DIORYTE. ( <!<thhrt<l. ) 



From a ridge south of No. 7.V>. 



Bef. Annual Report, x, pages 9G, 97; Bulletin ii, page 83. 



Mr//. A rather coarse-grained, granitoid rock, composed essentially of horn- 

 blende and altered feldspar. 



Mic. The section shows a rock which consists of secondary lim-nlk'nile and 

 rlt/<iriti; highly altered feldspar, and iron ore. The last frequently shows the opaque 

 gray alteration products common to iliufiii/i-. 



One section examined. 



Age. Keewatin. u. 8. G. 



No. 757. DIABASE (with olivine.) 



From a dike six feet wide, running northwest and southeast and cutting No. T.lti. 

 lief. Annual Report, x, page 96. 



Meg. A rather fresh diabase of medium grain. 



Mic. The section shows long lath-shaped plagioclases, fiuyife, <>liri)i<> and 

 iiniijiK-titr. The section is very noticeably less altered than those of the last two 

 numbers. A peculiar feature is that the olivine has crystallized at the same time 

 with and in part later than the feldspar. The areas of olivine are penetrated through 

 and through by feldspar, and frequently the intergrowth of these two minerals is 

 pegmatitic rather than poikilitic. 



Two sections examined. 



At/r. Probably Keweenawan. 



lii'tncii-k*. Compare Nos. 1275 and 1842, in which the olivine is also younger 

 than the feldspar. u. s. G. 



No. 758. DIABASE. 



From the top of the mountain and south of No. 757. This specimen is probably from the top of mount 

 Northrop, in N. W. } sec. 2, T. 64-6 W. 



Bef. Annual Report, x, pages 96, 97; Bulletin ii, pages 116, 117. 



Mi'!/. A dark greenish -gray diabase, of medium grain. 

 Mir. M. E. Wadsworth's description of this rock is as follows:* 

 "A grayish-green crystalline rock. In the section this diabase is seen to be much 

 altered. Its structure is ophitic, and the fniyite remains in places in distinct cores, 

 surrounded by green liornlilrmlc. Quartz, ririilife, rh/on'tr, hiotile, dctinoliti', tit<niif<> 



*Bullelin ii, pp. 110, 117. 



