472 THE GEOLOGY OB' MINNESOTA. 



[Oabbro. 



olivine, as well as the interference figures of orthorhombic crystals. They are also 

 positive, with strong double refraction. They are therefore referred to olivine. 

 However, their general appearance is slightly different in coloration, etc., from the 

 common basaltic olivines, and this is naturally the case since one cannot suppose a 

 mineral formed by secondary agencies to be identical with one formed primarily, 

 even if chemically and mineralogically they are the same. This olivine appears to 

 represent a case parallel to the secondary olivine of the St. Paul's rocks as described 

 by the present writer.* 



"Another alteration product is shown in the formation of scales and masses of 

 biotite, which partially replace the augites, which in most cases remain, forming an 

 elongated central band. The augite, in the earlier stages of alteration, is filled 

 with a brown dust and sometimes it is feebly pleochroic, the color varying from 

 chiefly brown to yellow. Apatite microliths are common, not only in the secondary 

 groundmass, but also transfixing the olivine crystals." 



We fail to see why Dr. Wadsworth regarded the olivine as secondary. It has 

 all the appearance of the original olivines of diabases or gabbros, and at times has a 

 tendency to assume a crystal form. The large feldspars are usually of earlier date 

 than the augite. They are evidently andesine-labradorite, showing equal extinction 

 angles in sections perpendicular to 010 as high as 21; moreover, a section cut almost 

 exactly normal to the least axis of elasticity gave an extinction of 12, and another 

 closely normal to the greatest axis of elasticity gave 64. 



One section examined. 



Age. Cabotian. 



Remarks. Perhaps a mixture of a basic and an acid magma, the latter being 

 represented by the groundmass of quartz and reddened feldspar. See remarks under 

 No. 649. u. s . G. 



No. 651. GABBRO. (Rich in magnetite, with olivine. ) 



"From boulders, seen along the same trail, twenty to thirty rods north of the last (No. 650). This same 

 rock forms a hill along the right of the trail." 



Bef. Annual Report, x, pages 68-70; Bulletin ii, page 90. 



Meg. A rather coarse-grained, dark-gray rock, evidently containing feldspar 

 and much magnetite. 



Mic. The section shows a rather coarse-grained granitic aggregate off<'/<Ixf><n: 

 magnetite, olivine and augite. The feldspar and magnetite are the most abundant 

 minerals, and the latter makes up fully one-third of the rock. The rock itself is not 

 attracted by the magnet and it is probable that the magnetite, like most of this 

 mineral in the Minnesota gabbros, contains a considerable amount of titanium. In 

 addition to the usual albite twinning the pericline twinning is common in the 



'Science, 1888, i, 590-592 ; Lithologtcal Studies, 1884, pp. 123-125. 



