192 



THE GEOLOGY OP MINNESOTA. 



[Labradorite. 



silica. After washing thoroughly, the remaining grains were all found to be stained 

 more or less but retained their forms, and in some of them the power of polarizing 

 light remained. It is probable that the finest grains were lost in the process of 

 washing. No distinctly gelatinous silica was observed. 



The edges and faces that are preserved are shown by the following figures, 

 which represent opposite sides of the twinned crystal, natural size: 



001 



poi 



021 



OQI 



FIU. 7. 



'00? 



FIG. 8. 



Angles were measured with a hand goniometer, on the faces, 010, as follows. In 

 most cases the faces are large and intact, affording reliable pose for the arms of the 

 goniometer. The exposed surfaces, 010, are positive in each twin, the contact and 

 twinning surface is 010. The obtuse angle 001 A 010 is conventionally at the top, 

 and at the right of the observer. 



