402 THE CRYSTAL FALLS lEON-BEARING DISTRICT. 



doubt was formerly an arkose rich in feldspar, has recrystallized and after- 

 wards been sheared; the coarse micas to which the fissility is due, together 

 with other new minerals, have grown between the fractured surfaces and 

 recemented the broken mass. It affords beautiful examples of foliation 

 parallel to a line. 



The thin sections of the Sturgeon quartzite are of exceptional interest. 

 The principal constituent is, of course, always quartz. With the quartz are 

 associated, in much smaller amounts, and not necessarily all in the same 

 section, numerous accessories, including muscovite, biotite, chlorite, micro- 

 cline, orthoclase, plagioclase, titanite, rutile, zircon, aj)atite, and the ores. 

 The relations of the quartz to the other constituents present very unusual 

 features, and indicate that the metamorphic changes by which the present 

 completely crystalline rock has been made from an original granitic sand 

 have proceeded along lines not hitherto distinctly recognized in the forma- 

 tion of rocks of this character. 



Among the large number of slides examined, a broad distinction can 

 at once be made between those which show the effects of stress in a pro- 

 nounced degree and those in which such effects are subordinate or hardly 

 noticeable. Connecting these two classes is a perfectly graded series; and 

 it is therefore certain that those of the first are merely the more or less 

 modified varieties of an earlier stage, represented more nearly by the second. 

 In the slides in which the effects of pressure are least apparent the micro- 

 scopic characters are as follows: The backgi'ound is composed of large 

 irregular grains of quartz, the edges of which interlock with the most minute 

 and sharp interpenetrations. The longest dimensions of these grains range 

 from 1.5 to 6 mm., averaging perhaps 2.5 or 3. They often have a rather 

 vague parallel elongation, which corresponds to the alignment of the minerals 

 which they inclose. Scattered very abundantly through these large quartz 

 grains are the accessory minerals, some predominating in one slide, others 

 in another, but the micas and chlorite occui'ring in all. Through each 

 slide the accessory minerals, with the exceptions noted below, lie with 

 their long axes in a common direction, and frequently cross the serrated 

 boundaries between adjacent quai'tzes. The inclusions in many cases have 

 the form and other characters of clastic minerals, and thus preserve the only 

 microscopic evidence of the original nature of the rock. 



The included micaceous minerals are usually in small plates, ranging 



