948 THE GEOLOGY OF MINNESOTA. 



[Sidorito. 



porphyries, so called (Nos. 387, 376), in the greenish conglomerates (Nos. 874, 908, 

 1070), in much of the mica schist (No. 422), in many diorytes (Nos. 424, 731, 1410, 

 1318j, and occasionally in granite (Nos. 730, 1100, 2263),and rarely in gabbro (No. 1802). 



Calcite shares with quartz the distinction of existing in all rocks except the 

 unmodified original massive greenstones and the similar basic igneous rocks of the 

 Keweenawan. It never, however, forms crystals of perfect form except in cavities 

 where it has room to develop without contact with other crystals (No. 60A) but it 

 is minutely disseminated widely where it can be detected only by the microscope. 



Siderite. The earliest known appearance of siderite is in the quartz-porphyries 

 of the Archean (Nos. 426, 428) and in the jaspilytes (Nos. 385, 388, 903, 907, 1565). 

 It is also a frequent minor ingredient in nearly all the associated rocks of the Kee- 

 watin, whether of the Lower or Upper (Nos. 319, 326, 389, 473, 747, 910, 911, 2266, 39S). 

 It appears uniformly as perfect, or approximately perfect, crystalline minute 

 rhombs, and it is not preferably associated with any of the other minerals except 

 hematite, with which it has an intimate connection. Sometimes these rhombs are 

 aggregated in groups (No. 903), in which case they are more correctly styled grains, 

 and exhibit that globular form and small dimension which is characteristic of many 

 minerals in an incipient state of crystallization in metamorphic rocks. When the 

 crystallization goes further, larger rhombs appear, and these larger crystals are seen 

 to surround one or several of the small globules, usually with different orientations. 

 Sometimes the whole rhomb seems to be made up of an aggregation of minute sid- 

 erite globules, but usually (as in No. 907) there are three or four distinctly outlined, 

 clustered or isolated, round which the darkened border due to the high refractive 

 index comes out markedly on lowering the condensing lens. These globules as a 

 rule do not break the borders of the rhombs, which are straight and extinguish simul- 

 taneously, but they lie somewhat away from the borders, not having exactly the 

 same orientation as the borders. 



These minute rhombs are also secondary to the minutely granular quartz which 

 usually, in the jaspilytes, constitutes the most of the rock. These granular masses 

 not only displace the quartz so as to make sometimes a spotted rock, of which its 

 quartz grains are of about the same size, but these patches surround and embrace 

 the quartz grains, and quartz grains are also to be seen, occasionally, in the midst of 

 the isolated rhombs. Indeed, the siderite acts the role of a poikilitic mineral, embrac- 

 ing the quartz. This is not conspicuously the case, but usually the siderite is inde- 

 pendent of the quartz. Yet it is so frequently seen that it is plain that the siderite 

 was developed later than the quartz, or was nearly cotemporary with it. These 

 siderite globules are easily distinguished from the quartz grains that lie within the 

 siderite by the use of the Becke white line, and by the marginal lines of color which 



