202 THE GEOLOGY OF MINNESOTA. 



[Diabase. 



easy cleavages. There is also a difference in the elongation of the fibres resulting 

 from initial disintegration. If the orientation can be determined, the interference 

 figure seen in 100 in enstatite is characteristically different from that seen in augite, 

 in the same face. 



The section of this mineral shows throughout a near perpendicularity to some 

 axis of elasticity. On searching further a portion is found near the margin of the 

 slide, in which this bisectrix is quite visible, and on testing it in the usual way with 

 the sensitive-tint quartz plate, it appears that the 'section is too thick to be suscep- 

 tible to the usual observation and comparison with Newton's scale. The direction 

 of the plane of the optic axes is found by this interference figure to be vertical to the 

 elongation of the fibres of the intercalated impurities. It is hence perpendicular to 

 the cleavage in which they lie. These fibrous impurities, however, find access, in 

 the first instance, along the cracks formed by a very irregular cleavage (?), spreading 

 out from these as they cross the fine cleavage. At the same time there is a third 

 cleavage, perpendicular to the elongation of the fibres, but this is not conspicuous. 

 It is, however, very straight and clear, and is visible best in faint light. Its lines are 

 short and interrupted, and are not everywhere discoverable. Its interrupted and 

 scant occurrence has operated to prevent, except in occasional instances, the entrance 

 and display of the fibrous impurities along its cracks. The great irregularity and 

 the coarseness of the oblique transverse cracks rather exclude them from the cate- 

 gory of cleavages, and there are left only the two rectangular cleavages parallel to 

 which extinction takes place. In the figure below, which represents the grain in 

 which the bisectrix is best visible, the second cleavage is hardly found, but some 

 irregular cracks, probably governed in their direction by it, appear perpendicular to 

 the fibrous cleavage. 



Mm. 



In order to determine which bisectrix is in the field of the microscope it is nec- 

 essary to resort to the mica of quarter undulation. Knowing the direction of the 

 optic axial plane, this plate is inserted in such a manner that its axis is parallel to 



