968 THE GEOLOGY OF MINNESOTA. 



[" Primary " and " secondary " minerals. 



2. "Primary " and " secondary " minerals. These conventional distinctions have 

 been found insufficient and inapplicable in numerous instances; though they have 

 been employed in the foregoing descriptions, it is only with some misgivings and 

 with a certain amount of recognized vagueness. In general, whenever these terms 

 have been employed, the "secondary" minerals designated are plainly those which 

 have resulted from alteration of other minerals which have been assumed to have 

 been "primary," or "original." 



Three different considerations have conspired to render it insufficient, and some- 

 times incorrect, to apply these terms to the minerals of a crystalline rock. 



1. There are three critical epochs in the history of an igneous rock, viz.: the 

 passage from the molten condition to the crystalline, i. e., the solidification, the epoch 

 of cooling subsequent to solidification when it is permeated by gases and suffers 

 rapid metasomatic alteration, and the epoch of atmospheric weathering, Circum- 

 stances may be such in the environments of the rock mass that either one of these 

 may be very long, and the changes produced very slow or very rapid. The 

 last mentioned, however, is quite insignificant in Minnesota. Any sample of rock 

 ordinarily procured in the field for examination will be found to pass below the 

 weather effects unless those effects have been abnormally intensified. The glacial 

 abrasion removed the weathering effects and left the surface fresh, and since the 

 glacial epoch, in the northern part of the state at least, the alteration of the 

 exposed minerals has been almost nil. It is only in exceptional conditions, 

 such as the local weathering of pyrite or other sulphide, or the oxidation of 

 some carbonate, such as siderite, that the minerals of the crystalline rocks can be 

 seen to be affected by post-glacial decay. Pre-glacial decay is sometimes preserved, 

 and, as will appear, even Archean decay, enters into the history of many rock masses. 



Therefore, in so far as the alterations seen in the igneous rocks of Minnesota 

 are able to be classified, they must fall within the two epochs first mentioned above, 

 and they must be referred to the agencies that were then in action. The first epoch 

 (the passage from the molten condition to the crystalline) is not an epoch of altera- 

 tion which can be said to produce secondary minerals in any sense. There is, there- 

 fore, nothing left but the cooling epoch which can be invoked for the explanation of 

 the occurrence of most of the secondary minerals in such rocks in the Keweenawan, 

 viz., quartz, hornblende, orthoclase, chlorite, hematite, most of the zeolites, probably 

 most of the apatite, bowlingite, most of the magnetite and all of the pyrite. In the 

 Archean igneous rocks no secondary minerals coordinate with the foregoing in 

 method or relative date of genesis are known. Considered only from the point of 

 view of Keweenawan history, it can be seen at once that, in ordinary usage, petrog- 

 raphers would differ widely as to the secondary or original nature of many of the 



