960 THE GEOLOGY OF MINNESOTA. 



[Actiuolite. Pareasite. Tremolite . 

 Uralite. Biotite. Chlorite. 



Actinolite, however, of all the fibrous, or fibre-lamellar amphiboles, is most 

 common. It is very abundant in all the green schists, and especially in the green 

 schists about Kekequabic lake. It is probably this mineral that prevails in all the 

 green, altered, basic elastics of the Keewatin, where it has frequently been passed 

 under the simple designation "hornblende." 



Pargasite differs optically from the other hornblendes in having n, for its acute 

 bisectrix (Nos. 1043, 1049). It is probably more common than has been observed, 

 especially about Kekequabic lake, where it occurs in the green tuffs of the region, 

 resulting, as already stated, from alteration of a soda-bearing augite. These horn- 

 blendes have not been analyzed, but it is very probable that they would show a small 

 percentage of soda. 



Tremolite has been named occasionally, but its optic characters are so similar 

 to those of actinolite that this distinction must be considered as provisional (Nos. 

 ISA, 1137, 1453). 



UraUte is the name that has been applied sometimes to a hornblende whose 

 dependence on augite as its source is very evident, sometimes in the preservation of 

 the original ophitic relation of the augite (Nos. 1386, 2255, 2258). 



Biotite. There seems to be an easy gradation in optic characters as well as in 

 chemical composition between muscovite and biotite. The alteration of the feld- 

 spars is the prime source of both. When such alteration is in the presence of ferro- 

 magnesian minerals likewise undergoing change, the mica partakes of the iron and 

 magnesia, thus affording biotite. A simple decay of the feldspars would hardly pro- 

 duce these results, but it must be understood that some agent is acting to promote 

 recrystallization. Such force may be heat, or pressure, or both, accompanied by 

 moisture, and the process may be slow or rapid. Without such forced recombina- 

 tions the soluble elements of the feldspars would be removed entirely, under ordinary 

 decay, and the result would probably be a pure kaolin (Nos. 1449, 1700, 1701, 1704). 



There are, moreover, instances in the igneous rocks (gabbro) in which biotite 

 appears to have been one of the original minerals (Nos. 291, 954). 



Muscovite and biotite, often in large crystals, are hence found widely in the 

 granites, diorytes and syenytes that have resulted from the recrystallization of 

 Archean debris. In a strictly pet rographical sense they are here original minerals, 

 but they are secondary in the broader sense that they have resulted from the decay 

 of earlier less stable mineral compounds, and have recrystallized under the stress of 

 metamorphic forces. 



Biotite occurs porphyritically in a kersantyte which acts as an intrusive in the 

 vicinity of Moose lake (Nos. 2158, 2261). 



Chlorite. Including under this term all the chlorites that have been observed 

 (clinochlore, pennine, strigovite, thuringite, ripidolite, delessite), the general remark 



