92 



THE GEOLOGY OP MINNESOTA. 



[Gabbr6. 



lamellation on 100 and ihe generation of magnetite date from the cooling period, 

 perhaps contemporaneously. 



Magnetite. From the above it appears that the genesis of the magnetite in this 

 rock is not all referable to the same date, but that it was both original and 

 "secondary." It is in quadratic forms embraced in the feldspars and in the diallages, 

 in comparatively unchanged surroundings. It fills angular openings within the 

 feldspars, where it seems to have taken the place of decayed pyroxenes (diallages), 

 and in this form it constitutes the largest amounts. At the time of this substitution 

 of magnetite for pyroxene, there was a general disintegration of the pyroxenes, so that 

 their spaces are not entirely filled with magnetite, but epidote, and especially chlorite, 

 appear as secondary minerals. Pyrite appears in a few small grains, mingled with 

 the secondary magnetite. 



FIG. 3. A GRAIN OF DIALLAGE SEEN IN SECTION OF ROCK NO. 1C, SHOWING THE PRISMATIC 



CLEAVAGES AND THAT PARALLEL TO 100. 



a Magnetite (black). b Feldspar. c Fibrous diallage. ch Chlorite. 



The rest of the figure is occupied by a grain of diallage. In this section the fine fibrous structure is wanting. 



The order of genesis of the minerals here seems to have been as follows: 



"Secondary," or without knoint 

 Original order of genesis 



1. Magnetite. Chlorite. 



2. Feldspar. Magnetite. 



3. Diallage. Pyrite. 



Epidote. 



In the magnetite sometimes there is a connection between a cubic or otherwise 

 angular grain, which is so situated with respect to a feldspar or a pyroxene as to 

 show its original date from the molten magma, and a mass of secondary magnetite, 

 as if the change of the pyroxene had been provoked to begin at a magnetite nucleus, 

 and to spread widely from it. In such a case the original cubic magnetite has a 

 sharp line of separation from the surrounding minerals, whether of pyroxene or of 



