966 THE GEOLOGY OF MINNESOTA. 



[Pyrite. The globular state of 

 incipient minerals. 



extending from the quartz-porphyries of the Lower Keewatin (No. 2238) to those of 

 the Upper Keewatin (No. 387), and from the Archean granite (No. 2248) to the red 

 rocks of the Keweenawan (No. 292), while in nearly all the Archean clastic rocks it 

 is frequent in form of cubic crystals (Nos. 340, 356, 473, 726.) It is so common that 

 it has not been considered necessary to mention it in many of the microscopical 

 descriptions. In connection with the jaspilyte ridges at Tower pyrite seems to exist in 

 considerable quantity, especially in the " south ridge," since its oxidation is palpably 

 the cause of water carrying sulphuric acid, which issues from a drift on the southern 

 side of the ridge. It was also encountered in the (Lee) mine, as evinced by Nos. 1547, 

 1549 and 2280. In many respects the environments as well as the crystalline perfection 

 of pyrite where seen in the Archean resemble those of magnetite, and there is 

 evidence that it originated in the Archean, where associated with magnetite, at about 

 the same date and by reason of similar or identical physical conditions. 



In concluding this sketch of the rock-forming minerals of the crystalline rocks, 

 it may be well to enumerate some general principles brought out forcibly by the 

 examinations detailed in Part II, but which are sometimes not recognized fully by 

 petrographers. 



1. The globular state of incipient minerals. That quartz assumes a globular 

 form at the commencement of its separation from a magma, and passes through 

 various structures by the extension or the multiplication of these units, was fully 

 announced with illustrations by M. M. Fouque and Michel Levy.* These incipient 

 and imperfect forms of quartz appear to arise under metamorphism and continue 

 their development under conditions which obstruct normal growth and especially 

 in the presence of other elements which simultaneously are stimulated to incipient 

 crystallization. 



In the course of this investigation several other minerals have been noticed to 

 undergo the same or similar succession of stages. Especially has it been noticed 

 that the globular form is acquired as the first step in the formation of minerals due 

 to metamorphism. -We have not had so favorable opportunity to study minerals 

 formed directly from magmas, although many dust-like globular forms have been 

 noted in the zirkelyte resulting from the Keweenawan eruptives. In the devitrifi- 

 cation of Keweenawan obsidians, the first forms developed seem to acquire at once 

 angular outlines or thread-like projections. 



In the rocks formed from metamorphism which did not reach fusion, it is so 

 common to observe the globular form in the first stage of all the minerals that it 

 seems to be a general law. Owing to the nature of the case this phenomenon is 



* Mi, i: /.,(,,,/,, j/iV/VM/ivi/)/ii</Me, 18T9, p. 193, plate xii, figure 1. The description and illustration given by the authors 

 hardly apply tolhe globular quartz here referred to, but to a more advanced state of growth. M. FouquS, however, in his lectures 

 at the College dc France, has amply discussed, the globular form of quartz here referred to. 



