546 THE GEOLOGY OF MINNESOTA. 



[Gabbro. 



Age. Cabotian. 



Remark. The section studied by Wadsworth was a very unfavorable one. 

 Having later had a better section prepared by Marchand, several features of this rock 

 are distinctly presented. (1) The olivine is in somewhat spreading masses with 

 smooth outlines, and somewhat embraces occasionally the labradorites whose outlines 

 are also rounded, thus resembling the olivine of rock No. 258. (2) The few magnetite 

 grains are surrounded by brown hornblende, rather than by biotite.* (3) The 

 border of pyroxene that surrounds the olivines is augite, with occasional close 

 twinning with some other pyroxene, and but rarely presents the lamellation charac- 

 teristic of diallage. Sometimes two growths appear in the same grain, the older 

 being corroded and surrounded by the later, and of different orientation. N. H. w. 



No. 788. GABBRO. 



East side of Burntwood lake; perhaps in N. E. % sec - 9> T. 62-4 W. The country rock. 

 Ref. Annual Report, x, page 104; Bulletin ii, page 101. 



Meg. A medium-grained, rusty, granitoid rock, composed of feldspar and black 

 minerals. 



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



"The section is composed of feldspar, augite, hornblende, biotite, quartz, apatite, 

 magnetite, etc. The feldspar is altered and kaolinized in part, and is both orthoclase 

 and plagioclase. The augite is of a brownish color, and along its borders, and some- 

 times throughout much of its mass, has been altered to a greenish fibrous and scaly 

 or leafy substance. This, in places, possesses the characters of green hornblende, 

 and in others those of biotite. The apatite and quartz are clearly secondary, both 

 occurring in the secondary hornblende. The augite and part of the magnetite and 

 feldspar are the only original minerals left." 



One section examined. 



Age. Cabotian. u. s. G. 



No. 789. GABBRO. 



Portage from Burntwood lake (probably) to Temperance River lake (called Descent lake on the map, 

 plate I of the Tenth Annual Report); perhaps in W.* sec. 10, T. 62-4 W. 

 Ref. Annual Report, x, page 104; Bulletin ii, page 87. 



Meg. Very similar to No. 783. 



Mic. The section shows an altered gabbro, the augite being largely replaced by 

 hornblende and the feldspar being much kaolinized. Quartz and apatite needles also 

 occur. 



One section examined. 



Age. Cabotian. u. s. G. 



* Brown hornblende, having its maximum extinction angle sometimes as low as one degree, and being highly pleochroic in 

 the same colors, is easily mistaken for biotitu. 

 ^Bulletin ii, p. 101. 



