322 THE GEOLOGY OP MINNESOTA. 



[Dioryte. 



crystals. Slender spicules which are associated with the feldspar seem to consist of 

 (t[ndite, while epidote is quite common. 



The existence of quartz in this rock is very rare. The feldspars have the 

 appearance of quai'tz, being clear and limpid in extinctions as the stage rotates. 

 These grains, when tested for the characteristic interference figure, uniformly give 

 a biaxial sign, and on lowering the nicol they are seen to contain numerous 

 enclosures, and sometimes show a cleavage or a structure indicating feldspar. In 

 one such instance the bisectrix (n g ) gave an extinction angle of 8, which is near 

 andesine. Another exactly perpendicular to n p gave an extinction angle of 66, 

 which exactly agrees with andesine. 



One section. 



Age. Archean (Coutchiching). 



Hcninrk. The transition from the igneous rock of the "Laurentian" of the 

 region of Bassimenan lake to the Coutchiching at this place is no less noticeable 

 petrographically than.it is outwardly. The abundant quartz of the light colored 

 igneous Laurentian disappeai-s almost entirely, and the clouded, semi-saussuritized 

 oligoclases, microclines and orthoclases, give place to a glass-clear andesine. 



N. H. w. 

 No. 334. DIORYTE. 



Same locality as No. 332. "Chloritic hornblende schist, conformable with No. 333." 

 Ref. Annual Report, ix, page 88; Bulletin ii, pages 87, 88, plate VIII, figure 2. 



Meg. A dark, almost black, medium-grained rock composed very largely of 

 hornblende, with some gray feldspar. 



,l/7r. The section shows hornblende in abundance, feldspar, epidote and sphene. 

 The feldspar is sometimes very highly altered and sometimes quite fresh. In the 

 highly altered portions much epidote has been developed. The fresher feldspar 

 frequently shows fine twinning lamellae and a low extinction angle -and probably 

 is near oligoclase. Many of the smaller grains do not show twinning, but still seem 

 to be plagioclase. An untwinned grain cut almost exactly perpendicular to the 

 positive bisectrix shows an extinction angle of about 3, indicating (/itd<'xi>tr-o/!f/oclase. 

 (A section of orthoclase cut parallel to the brachypinacoid would show the positive 

 bisectrix and a low extinction angle, as would also a similar section of andesine- 

 oligoclase. In this case, however, it is probable that no orthoclase is present.) 



Remark. In Bulletin ii, a quartz-pseudomorph after plagioclase is described and 

 figured. This very grain, however, seems to be feldspar and so do the surrounding 

 smaller grains. This grain and many others which seem to be quartz on casual 

 examination, were tested and many of them were found to show a distinct biaxial 

 character, and not one showed clearly a uniaxial nature. There were, however, a 



