858 THE GEOLOGY OP MINNESOTA. 



[Peridotyte. Magnetite iron ore. Muscovadyte. 



Mic. There is a little quartz and many remnants of o\& feldspars that have not 

 been entirely consumed by the micro-granulation which has permeated the rock 

 and which is much finer than in No. 2187. Hornblende and biotite compose the dark 

 elements with a very little magnetite and sphene. Two sections. 



Age. Archean (Keewatin). N. H. w. 



No. 2199. PERIDOTYTE. 







Same place as the last, but further south, at the iron-ore pits. 

 Ref. Annual Report, xxiv, page 62. 



Meg. The rock that embraces the ore, grading into the ore- 

 Mic. Olivine, diallage twinned with enstatite, quartz, magnetite, in the order 

 named, compose this rock. The twinned enstatite embraces all the others poikilit- 

 ically. There is a tendency to roundish outlines, especially for the olivine and the 







quartz. That this contains enstatite rather than hypersthene is shown by the 

 low double refraction (about the same as quartz), and by a basal section showing 

 with the prismatic cleavages another parallel with the optic plane, and also showing 

 the bisectrix n t in the acute angle, which latter is quite small.* Probably the more 

 intense contact with the gabbro at this point, or perhaps the presence of a large 

 supply of iron, had something to do with this difference in the ferro-magnesian 

 minerals. The olivine has the small optic angle (containing ) characteristic of 

 fayalite. One section. N. H. w. 



No. 2201. MAGNETITE IRON ORE. 



Same place, Disappointment lake. 

 Ref. Annual Report, xxiv, page 62. 



Meg. Appearing siliceous. 



Mic. About one-half this rock is magnetite. The larger part of the rest is 

 quartz. But there is a noticeable amount of the ferro-magnesian elements seen in 

 Nos. 2198 and 2199, particularly diallage. The magnetite forms a sort of sponge in 

 which the other minerals are held in roundish kernels. One section. 



Age. Archean (Keewatin). N. H. w. 



No. 2202. MUSCOVADYTE. (Noryte.) 



Thirty feet further south than No. 2201. Disappointment lake. 

 Ref. Annual Report, xxiv, page 62. 



Meg. This cuts the conglomerate which contains the iron ore, and embraces 

 pieces of the conglomerate. 



Mic. The pyroxene is hypersthene, but it is interleaved parallel to 010 by a 

 monoclinic pyroxene, which has a higher birefringence, probably diallage. This 



* Miniraux des Roches, p. 260. 



