194 THE GEOLOGY OP MINNESOTA. 



[Aporhyolyte. 



Meg. Similar to No. 127, but porphyritic with orthoclase (?) and with quartz. 

 The outcrop has no evenly laminated arrangement, but is frequently jointed and 

 easily falls to pieces. 



Mic. The fehlxpars and (jnt<rt.zcs are about equally abundant, and each so 

 distributed that there is one in the area of about one-eighth of a square inch, the 

 former being flesh-colored and sometimes a quarter of an inch in length and the 

 latter about one-sixteenth of an inch across, with rounded outlines. They are 

 embraced in a light-purplish matrix which evidently was originally glassy. This 

 matrix is now filled with a multitude of rounded opaque microliths probably con- 

 sisting of iron oxides, with scattered light-yellow grains of irregular shapes, having 

 a strong refractive index which are probably of sphene. Sometimes these grains of 

 supposed sphene occur in the quartz phenocrysts, but generally they are scattered 

 through the matrix. In the case of their existence within the earlier quartz, it is 

 apparent, in the single instance observed, that the quartz was broken, and some part 

 of the glassy magma entered in that way; this became later the gathering place of 

 the titanium mineral. In the same quartz crystal is seen a single apatite spicule (?) 

 but this has no connection with the fracture plane mentioned. It has irregular per- 

 pendicular cross-fractures, and between the nicols it darkens when parallel with 

 either thread. It presents colors of birefringence, though the section is less than 

 .003 millimeters in thickness, and hence it may not be apatite. In the devitrified 

 matrix, however, are other apatite spicules. 



Three of the quartzes, illustrated below, are so situated as to indicate that they 

 were probably at first embraced in one crystal. They have the same aspect between 

 crossed nicols in the manner of distribution of lines of minute impurities. That 



marked a has an interference figure consisting of a dark 

 bar, which, spreading considerably as it crosses the field, 

 seems to become curved as it leaves, in either direction, 

 the concave side being in advance, in the direction in which 

 the bar moves, and hence the first part of the bar to leave 

 the field. This is characteristic of a uniaxial crystal, especially of quartz, cut 

 very obliquely to the optic axis. That part marked b is apparently perpendicular to 

 a bisectrix of a biaxial crystal, /. e., two dark hyperbolic curves approach each other 

 from opposite quadrants and unite to form a dark cross in the centre of the field. 

 But this phenomenon is not necessarily demonstrative of the presence of a biaxial 

 crystal since a thin section of quartz cut parallel to the optic axis affords the same 

 interference figure. That part marked c has an interference figure like that in a, but 

 more rigid and darker. It indeed has the aspect of the recurring arms of the black 

 cross of a uniaxial crystal. The direction of extinction in each part is marked by 



