942 THE GEOLOGY OP MINNESOTA. 



[Quartz. 



been attributed to decay since the formation of the rock in which they are found, 

 and the microcline structure to unequal strains in orthoclase caused by mountain 

 pressure. As the two facts are frequently concomitant, such decay and such unequal 

 strains have been believed to be complementary and nearly cotemporary. 



Whatever may be the causes of such phenomena in other places, we have seen 

 no reason to attribute them to recent decay nor to mountain pressure in the rocks 

 we have examined from the Archean in Minnesota. Indeed, we place them at the 

 opposite end of mineral genesis. The minerals presenting these features are fresh 

 and chemically intact. They had passed through a primordial period of decay, but 

 they are now fresh and pure, and the elements of that earlier decay are rejected by 

 the new crystallization. The following considerations seem to preclude the idea that 

 this alteration is due to recent decay, or even to decay since the present solid con- 

 dition was acquired by the rocks concerned. 



1. No such alteration is known since the Glacial epoch. 



2. This alteration is not superficial, but universal and at all depths examined. 



3. It is not marginal on the grains, but central. 



4. It is not in the midst of decaying rock materials, but in minerals having a 

 fresh and strongly interlocked granitic texture. 



5. The rims that extend beyond the old borders are fresh and sometimes even 

 glassy in their transparency. 



6. The same distinctions can be seen in some porphyrel which is unquestion- 

 ably a partially recrystallized clastic containing many large old feldspars, as well as 

 in some graywackes. 



7. The other elements, such as hornblende orbiotite and muscovite, are wholly 

 fresh. They seem to date from the generation of the new feldspar and quartz. 



8. These elements, with sphene and epidote, are evidence of some former period 

 of alteration, and their freshness denotes a later period of recrystallization. 



9. Such central "alteration" is not seen in the diabases norgabbros, nor in the 

 greenstones, nor in the most of the graywackes and conglomerates, but in these 

 rocks the feldspars, when partially decayed, are evenly sprinkled with the products 

 of such change, or are chiefly altered about their margins. 



In the " red rock " series a substance occurs abundantly which has frequently 

 been assumed to be orthoclase, but which in some instances is a semi-de vitrified acid(?) 

 glass and in others is the result of change of some plagioclastic element. The former 

 is most frequent in the aporhyolites and the latter in the contact rocks of the red 

 series with the diabases (No. 686). In some instances there is left enough of the 

 albite twinning to show that the feldspar, which is perhaps reddened by hematite, 

 is really an altered plagioclase (Nos. 42, 45, 850). Sometimes, apparently, consider- 



