944 THE GEOLOGY OF MINNESOTA. 



[Bytownite. Anorthite. Anorthoclaae. 



Aside from the Keweenawan, however, labradorite is comparatively rare in 

 Minnesota. It is found in connection with the rocks intermediate between the 

 gabbro and the greenstone of the Keewatin, i. e., in the muscovadytes, where it is 

 often associated with very unusual companions, but in other parts of the Keewatin 

 it is so modified by decay or regenerated by subsequent new growths, or is rendered 

 so indefinite in its optical characters that it may be said to be practically wanting. 



Labradorite in the Keweenawan shows a peculiarity which is not confined, 

 however, to this mineral -in having not only two dates of generation, i. e., that of 

 the " first consolidation " and that of the second, but different relative dates as to 

 the accompanying minerals. It is sometimes earlier than augite, as in all the ophytes 

 and in some portions of the gabbro, as the latter term has been used, and is some- 

 times later or cotemporary with augite. The former structure is illustrated by figures 

 2 and 12 of plate I, rocks Nos. 108 and 820, and the latter by figure 4 of plate I, rock 

 No. 122 and by rock No. 137. It also sometimes occurs that the same rock section 

 shows two dates of labradorite, with respect to the augite associated with it (Nos. 

 802 and 2064, plate V, figure 6). Labradorite is usually later in origin than olivine, 

 in the basic rocks of the Keweenawan, but there is a large central area of the gabbro 

 mass in which it crystallized earlier than the olivine. This rare structure is illus- 

 trated by figure 7 of plate I, rock No. 258, and by figure 1 of plate V, rock No. 1829, 

 and it has been noted in numerous other rocks, viz., Nos. 512, 560, 603, 703, 787, 819, 

 1275, 1828, 1829, 1842. 



In the muscovadytes labradorite -sometimes embraces poikilitically the other 

 minerals and sometimes it is in globular small grains embraced in them. The anor- 

 thosyte masses of the Keweenawan are composed, so far as observed, of labradorite 

 (Nos. 113, 128, 200A, 223, 637). 



Bytownite, for petrological consideration, is to be classed with labradorite, and 

 may exist in the Keweenawan in greater frequency than is known. Sometimes it 

 has been identified distinctly (Nos. 770, 810, 814) and in other cases it seems to be 

 labrador-bytownite (Nos. 128, 258). 



Anbrthite, which is theoretically a pure lime feldspar, has been but doubtfully 

 recognized in the Keweenawan (Nos. 133, 176, 222, 637). An impure anorthite was 

 discovered in a fragment in clastic greenstone (No. 1367&). 



Anorthodase. This mineral, which can be considered the result of a variable 

 combination of the molecules of orthoclase and of albite, has been identified several 

 times in the rocks of the red rock series, especially in the granites (Nos. IB, 292, 

 511). It occurs also as a porphyritic constituent in the red aporhyolyte of the 

 "Great Palisades," No. 140(7), and in the associated granophyres at Pigeon point.* 

 The same feldspar has been identified with more or less certainty in the granites of 



*W. 8. BATLKT. The eruptive and sedimentary rocks of Pigeon Point, Minnesota. Bulletin clx, U. S. Geol Survey, p. 62, : 



