4 9 '2 THE GEOLOGY OF MINNESOTA. 



[Gabbro 



extinction and a negative elongation. They are regarded as feldspar and may be 

 orthoclase elongated in the direction of the inclined axis (a). In places there are 

 areas between these spherulitic masses which are filled in with a reddened substance 

 (feldspar), the iron ores, quartz in distinguishable grains, and chlorite. 



Scattered all through the section are yellowish-green needle-like bodies which 

 have little or no effect on polarized light. At times they assume a feather-like form. 

 They are thought to represent original crystalliths, possibly of augite. 



Two sections examined. 



Age. Cabotian. 



i!<-iii<trl>*. Compare Nos. 658 and 686. u. s. o. 



No. 688. GABBKO. 



Cross lake; from the outcrop first seen on the right when coming from the portage between Misquah and 

 Cross lakes; N. W. M sec. 32, T. G4-1 W. 



Ref. Annual Report, x, pages 78, 82; Bulletin ii, pages 88, 89. 



Mi'/j. A gray, rather coarse-grained rock consisting of grayish and pinkish 

 feldspar, pyroxene and magnetite. On the weathered surface most of the feldspar 

 is white. 



Mic. M. E. Wadsworth's description of this section is as follows:* 



" Under the microscope this rock is seen to have its ditt/laye rnnch altered, 

 principally to a uralitic product. The plagioclase, in places, exhibits its twinning 

 only at the point of extinction, as was shown by the writer to be the case with the 

 plagioclase of the Bishopville meteorite.f 



" The feldspar is also much altered and the section in places shows the structure 

 of graphic granite or the eozoon character occurring in the granites and felsites of 

 Keweenaw point. In No. 688 this structure is evidently produced by the aggregation 

 of the silica in the midst of the feldspar through the medium of the percolating 

 waters, while the remaining feldspar itself is much changed and stained by ferruginous 

 material. Much Hjxifite occurs in the section. " 



One section examined. 



Age. Cabotian, u. s. G. 



No. 689. GABBKO. 



This rock makes the shore and islands of Cross lake. Probably from sec. 2!i, T. (M-l W. 

 Ref. Annual Report, x, pages 78, 79; Bulletin ii, page 7.">. 



Meg. A light-gray, rather coarse-grained rock composed of white and a little 

 pinkish feldspar, augite and magnetite. 



Mic. M. E. Wadsworth's description of this rock is as follows :+ 



*Bullelln ii, pp. 88, 89. 



i-.-l iin-rii-an Junrinil /./ fti-ii'iice, 1888 (3), xxvi, 34; Lith<:l>aji<;il stiulirs, 1884, p. 200. 

 ^Bulletin ii, p. 75. 



