194 THE CRYSTAL FALLS IRON-BE AEING DISTRICT. 



is nearly always extensively altered, while the outer zones are compara- 

 tively fresh. A maximum extinction angle of 15° against the twinning 

 planes in the zone perpendicular to 010 was observed on an unaltered zone 

 surrounding an altered core. This would indicate the feldspar to be per- 

 haps as basic as labradorite at the center. The other essential minerals, 

 quartz and orthoclase, occur in usual quantity and show nothing of espe- 

 cial interest. Sphene and apatite are the only accessor}- minerals present. 

 An apatite crystal was observed which was included in quartz, and con- 

 tained the brown apparently vitreous core so frequently seen in the apatites 

 of basic rocks. Where included in l^iotite, it is surrounded by a pleo- 

 chroic halo. 



No analyses were obtained of these granites, but from the quantity 

 and character of the feldspar as noted above, these rocks are thought to be 

 closely related to dioritic rocks. Indeed, it is a, (juestion if they should 

 not be classed as quartz-diorites. 



RELATIONS OF GRANITES TO OTHER INTRUSIVES. 



In two cases already mentioned granite dikes cut the diorites. Granite 

 also cuts the gabbro, and dikes of granite were observed penetrating the 

 dolerites, thus indicating that the granites are young-er than these igneous 

 rocks. 



DYNAMIC ACTION IN GRANITES. 



An examination of the granites with particular reference to pi-essure 

 phenomena shows that they exhibit a great difference in this respect. Some 

 show scarcely any traces of pressure, while others quite closely associated 

 may have been aifected thereby to such an extent that a more or less 

 strongly wavy extinction of their mineral constituents is general. However, 

 but a single instance of a supposed granite possessing- an excellent cataclastic 

 structure and imperfect schistosity was observed, and this rock was so 

 extremely altered as to render doubtful a determination of its original 

 character. 



CONTACTS OF GRANITES AND SEDIMENTARIES. 



The largest intrusive granite mass is found between the Paint and 

 Michigamme rivers, in sees. 19, 20, 29, and 30, in T. 42 N., R. 31 W. 

 The granite is a muscovite-biotite-granite. The sedimentaries are mica- 

 ceous graywackes, which have been described on p. 170. Let it suffice 



