660 



THE GEOLOGY OP MINNESOTA. 



[Esterellyte. 



rarely interlock. Yet, that they grew together in the same rock, in same place, is 

 evident from the inclusion of minute augites occasionally in the feldspars. That 

 they are both out of their native places is further evident from the nature of the 

 surrounding matrix, which consists of debris of actinolite and of secondary quartz, 

 combined with fragmental stuff largely made up of feldspathic and augite grains. 

 Such feldspars, holding augite in the same manner, are seen also in the porphyrel 

 of Zeta lake, which is plainly composed of clastic materials, and is believed to be a 

 less metamorphic analogue of this rock. 



The following account by Dr. Grant, of the augite of this rock, No. 1094, which 

 is the same as No. 86G, is taken from the Twenty-First Annual Report, pages 45-48. 

 "The augite occurs in short, stout prisms, whose length is half a millimeter or 

 less; rarely, larger crystals, one to three millimeters in length, are seen. The crystals 

 are generally completely idiomorphic, but occasionally the terminal planes are lacking, 

 or are very poorly developed. The prismatic planes are the unit prism, the ortho- 

 pinacoid and the clinopinacoid. The terminal faces, which are usually present, 

 are the basal plane and the orthodone P^T while the unit pyramid and a clinodome 

 can sometimes be recognized, but usually there is a tendency to a rounding off of 

 the edges of the basal plane and the orthodome PST. The cleavage is well developed 

 in thin sections and parting is usually not seen, but in one case (see figure 43) it is 

 quite noticeable. An attempt was made to measure the angles on some of the 

 larger augite crystals detached from the rock, but the faces gave such imperfect 

 reflections that no satisfactory results were obtained. 



"In transmitted light the augite is of a bottle green color, but there are parts of 



some crystals which are colorless and entire colorless 

 individuals are sometimes seen. A slight pleochroism 

 is to be noticed in many sections, a and b being bottle 

 green and not distinguishable from each other, while c 

 is a yellowish green. The absorption is a=b>c. 



" Zonal structure is rather common ; in such cases 

 the core of the crystal is usually colorless, or of a 

 lighter green than the outer rim. The colorless 

 centres occasionally pass gradually into the colored 

 rims, but generally the two are separated by a pretty 

 distinct line. The outlines of these colorless cores are 

 irregular and are seldom parallel to any crystallo- 

 graphic planes. The cleavage lines run uninter- 

 ruptedly from one part of the crystal to another, and 

 in sections cut parallel to the zone of the ortho-axis 

 the extinction directions of both parts of the crystal 

 are parallel, but in sections which are inclined to the 



FIG. 42. DIAGRAM SHOWING THE REL- 

 ATIVE POSITIONS OF THE ORYSTAL- 

 LOGRAPHIC AND OPTICAL AXES IN 

 THE GREEN AND COLORLESS AUGITE. 



