116 THE GEOLOGY OF MINNESOTA. 



[Diabase. 



Mic. The feldspar has an average extinction, on 010, on a cleavage, of 25 to 28. 

 On the face 010, in convergent light a curved dark bar is the only portion of the 

 interference figure that is visible. Most of the grains are in the form of lath-shaped 

 twinnings, elongated parallel to the edge 001: 010; and extinction is not coincident 

 on all portions of the separate lamellae. This indicates some deformation since con- 

 solidation. By the statistical method the highest equal extinction angle on opposite 

 sides of a line separating two twins is found to be 19. This result agrees with the 

 foregoing measurement on the cleavage in indicating labradorite, although the 

 measurement on the cleavages seen in 010 seems to show a tendency toward bytownite. 



After some search, some sections of this feldspar can be found showing, in 

 convergent light, optic axes and bisectrixes. In the latter case, in two instances, it 

 proves to be n f (c) in one case not in connection with distinct cleavage, but in 

 another on a face showing evident cleavages parallel with the striations, indicating 

 that the section is nearly on 010. It is therefore the positive bisectrix appearing 

 somewhat obliquely in the brachypinacoid, which is characteristic of the labradorites 

 and bytown-labradorites. Many of the microlites are crossed nearly at right angles 

 by a pericline twinning. 



The pyroxene was later in crystallization than the feldspar, and embraces the 

 feldspar. It extinguishes over considerable areas, often involving several detached 

 parts. It is quite fresh, in some grains. The optic plane makes an angle of 4 to 5 

 with a distinct cleavage, and with a coarse parting an angle of about 40. These 

 measurements are made on the straightened dark bar crossing the interference figure 

 of a single optic axis. On this figure, in blue light, the curved dark hyperbola is 

 further dispersed than the same in red light, hence the red color lines the concave 

 side, indicating, for this axis p < v, but as the dispersion in the monoclinic pyrox- 

 enes is inclined, these colors may be reversed in the interference figure, in the manner 

 of their position on the hyperbolas. 



After some search a small crystal of the augite is found perpendicular to a 

 bisectrix. Its form is roughly that of a prismatic section, with a cleavage that nearly 

 agrees with the position of extinction. This cleavage must therefore be that parallel 

 to the brachypinacoid. Other cracks cannot be identified with certainty with any 

 cleavage. In other sections nearly rectangular cleavages are frequent. The optic 

 axes are carried beyond the field of the microscope, and are not brought within it by 

 the use of methyl iodide as immersion liquid. This bisectrix is f (c), as shown by 

 the use of the teinte sensible. 



The olivine is almost entirely changed. The change consists in the formation 

 of both serpentinous fibres and a great many minute crystallites of quartz, and 

 calcite and of a ferruginous dark substance that may be magnetite. The olivines 



