940 THE GEOLOGY OF MINNESOTA. 



[Quartz. Orthoclase. 



rocks and in the igneous rocks that have resulted from them through intense heat 

 and pressure in the presence of moisture. 



From this view of the origin of quartz, it follows that it is of secondary date in 

 pegmatyte and micropegmatyte (Nos. 643, 672, 686) in the quartz mosaic that results 

 from the devitrification of volcanic glass, whether acid or basic, and in the apobsidians 

 and aporhyolytes of the Keweenawan. It also follows that in all rocks resulting from 

 the crystallization of fragmentals, whether schists, gneisses or granites, quartz is in a 

 secondary condition, however compactly and intimately it may be interlocked with 

 the other minerals. The manner in which quartz develops poikilitically in the for- 

 mation of granite from clastic debris is illustrated by rock No. 1039. 



It follows, also, that there is very little "original" quartz. The only original 

 minerals, in a broad sense, are those that composed the original and oldest rocks, 

 viz., the massive greenstones of the Kawishiwin, or the oldest quartz-porphyry and 

 its allies of the Lower Keewatin. Original quartz occurs in the latter but not in the 

 former. By manifold chemical and mechanical transformations and perhaps by later 

 chemical oceanic precipitation, the quartz that permeates all the later formations 

 has been derived, whether those formations be clastic or igneous. 



In a restricted sense, however, the term original is often applied to all the 

 minerals of a rock perfectly and freshly crystalline, especially if it cooled from a 

 molten condition, and the term secondary to those minerals that have resulted from 

 the decay or alteration of the original minerals. There is, however, a degree of uncer- 

 tainty and vagueness in the use of these terms in this sense, owing to the blending of 

 the effects of original crystallization and of metamorphism and recrystallization. 



Orthoclase is everywhere an accompaniment of quartz, and nearly all that 

 has been stated regarding quartz is equally true of Orthoclase. In one respect, 

 however, they differ, viz.: an "altered" or a recrystallized grain of Orthoclase is easily 

 distinguished from an original one. In the quartz-porphyries of the Lower Keewatin 

 (Nos. 2229, 2237, 2238) where quartz first appears, Orthoclase also is found, but the 

 crystals which it forms are fragments!, resorbed and clouded by alteration. They 

 have the appearance of having suffered much abrasion and decay before they were 

 finally embraced in solid rock. They are decayed throughout, evenly, as evinced by 

 the uniform dissemination of many muscovite scales. While such a crystal is evidently 

 original in all senses of that word, it is evidently not in its original condition. Pure, 

 fresh orthoclase is free from such muscovite scales, and from all other alteration 

 products, and is nearly as clear as quartz. So long as such a crystal of orthoclase 

 maintains that flecked appearance evenly distributed through its whole mass it may be 

 considered as an original mineral. But when, in a later period of its history, the 

 rock mass in which it lies is subjected to metamorphic forces, becomes plastic, and 



