426 THE GEOLOGY OF MINNESOTA. 



[Diabase. Epidote. 



they occupy the whole field of the microscope. In one grain the intersecting 

 twinning is rectangular, and the form revealed is quite like the quadrillage of micro- 

 cline. This section is parallel to the base. In another grain the section is oblique, 

 and the interpenetrating twinning is not so close. Three cleavages are brought into 

 view conspicuously, which intersect so as to form isosceles triangles throughout the 

 section. These grains are, in general, brown, but they have areas between the 

 cleavages that are sub-translucent. This rutile is no doubt an alteration product 

 a-fter ilrnenite. 



.\uijitc is abundant. It shows the anomaly of being comparatively fresh, while 

 all the other original minerals (excepting apatite) are much changed. It crystallized 

 early, in part at least, for some of the grains are perfect crystals, and, cut perpendic- 

 ular to the vertical axis, show their characteristic cleavages and peripheral faces 

 and angles. Other grains formed after the feldspars, and these are more altered. 



Three sections. 



Age. Manitou. N: H. w. 



No. 568. DIABASE (with olivinej. 



Saginaw mine, near Conglomerate bay, Isle Roj'ale. 

 Ref. Annual Report, x, page 53. 



Meg. A rather fine-grained, dark -greenish, gray rock, apparently considerably 

 decayed. The only minerals noticeable are a greenish gray one (feldspar) and a 

 black to reddish glistening one. 



Mic. The section shows a rather fine-grained olivine diabase, somewhat decayed. 

 The au-gite is violet to straw colored, is sometimes very slightly pleochroic, and 

 frequently occurs in plates of considerable size holding many of the small j'rt</xj><i r 

 laths. The olivine is now completely altered to chlorite. Filling in what were 

 apparently cracks in the original olivine grains is a reddish-brown material. The 

 whole section is more or less altered and now contains alteration products, chief 

 among which is chlorite. Iron ore (ilmenitc or iinigiietite) is quite common. 



One section. 



Age. Cabotian(?) u. s. G. 



No. 569. EPIDOTE. (Rock). 



Saginaw mine, near Conglomerate bay, Isle Royale. This rock is the ore of the vein and was mined for 

 copper. 



Ref. Annual Report, x, page 53. 



Meg. A mass of epidote, with some quartz and considerable copper. 



Mii\ The section is in large part composed of epidote. In many places are 

 small areas which appear like a fine-grained diabase or similar rock composed of 

 small feldspar laths in a sparse groundmass of altered augite or glassy material. 



