970 THE GEOLOGY OP MINNESOTA. 



[Tertiary. Quaternary. 



5. Another class of minerals, which may have any date subsequent to those of 

 No. 3 above, are due to dynamic metamorphism in presence of moisture. 



The feldspars, labradorite and bytownite, would be primary, when porphyritic, 

 also olivine, some apatite and some magnetite. 



The most of the rock, including most of the feldspars, usually the augite, occa- 

 sionally olivine and perhaps biotite, and basaltic hornblende, would come second in 

 order, and in this scale would be secondary minerals. 



Tertiary minerals would embrace most of the magnetite, the orthoclase, quartz, 

 apatite (in large part), chlorite, bowlingite, epidote and any amphibole. 



Quaternary minerals would be such as calcite, limonite, chlorite, sericite, quartz 

 and perhaps some of the zeolites, and numerous others. 



Minerals of the fifth class are allied in cause and in kind to those of class No. 3, 

 and also embrace some of classes 1 and 2, since metamorphism may be partial or 

 complete and may take place with or without great pressure. When under great 

 pressure the history begins again de novo and the primary minerals are reproduced. 



These steps in the history of mineral genesis in the crystalline rocks are natural 

 and essential to be recognized. They include all that has been seen and recorded 

 in the mass of special descriptions foregoing in this volume, and theoretically might 

 be expanded so as to embrace all minerals; yet the descriptions have not usually 

 taken these steps into account. 



2. The acid igneous rocks, most of which are in the Archean, constitute a very 

 different category as to origin, and necessarily all their minerals share, each with its 

 own ontogenesis in the characteristics of that different history. It is here that the 

 terms original and secondary are specially inapplicable, for in one sense the minerals 

 are all secondary, and in another they are primary, while in another they would 

 mostly fall into the fifth class above. As will be seen in the discussion of the rock 

 masses as such, the Archean acid igneous rocks are traceable to an earlier clastic 

 condition. In many cases the remains of those clastic grains are preserved, and in 

 many others they are so regenerated that but little or no trace of them can be 

 separately detected. They acquire new forms, larger sizes and different composition, 

 in whole or in part. Whether the term original should be applied to the earliest 

 clastic condition recognizable, or to the partly or wholly regenerated condition, 

 er to the earlier non-clastic condition of the (later) clastic state of the separate 

 grains, is a matter which each petrographer would have to decide arbitrarily for 

 himself, and in deciding which he might differ from his fellows. 



3. Many metamorphic rocks pass into truly igneous rocks the gneisses to 

 granites and diorytes, the muscovadyte to gabbro. If principles be adopted for the 

 application of the terms original and secondary to the minerals, say of a gabbro 



