938 THE GEOLOGY OP MINNESOTA. 



[Quartz. 



It is obvious, therefore, that in many cases it is wholly impossible to state 

 whether a grain of quartz seen in a given rock section is original or secondary, in 

 the usual petrographical signification of those terms. There might be discovered, 

 perhaps, by a very exhaustive study of the internal structures of quartz grains, and 

 of the distribution of their minute inclusions, such differences between the original 

 clastic grains and those grains which have been wholly rewrought under metamorphic 

 agencies, that they could be distinguished, the one from the other. But we have not 

 attempted any such examination. The fact that a quartz grain has been developed 

 by new growth to fill the surrounding angular spaces, or that it encloses the other 

 minerals of the rock, such as feldspar, mica or hornblende, has been considered 

 sufficient indication of the later date of the quartz. In some such cases the borders 

 between the old and the new quartz remain, marked by a more or less distinct band 

 of ferruginous inclusions. Such borders, however, are found only in the least 

 metamorphosed rocks. In far the greater number of instances the quartz of the 

 crystalline rocks, whether metamorphic or igneous, is so completely changed that 

 no trace remains of its original shapes. 



Under metamorphism, quartz is quick to make its appearance. This seems to 

 result in part from the dissolution of unstable compounds, such as volcanic ash, or 

 undifferentiated volcanic glass, and in part from the superficial evaporation of silica- 

 bearing alkaline waters. Fresh waters penetrating the rocks become alkaline. 

 When heated, these waters not only take up silica, but also rise gently toward the 

 surface, and, there evaporating, they part with their surplus silica, rendering the 

 rocks through which they pass slightly more quartziferous. Thus veins are filled, 

 and, to a limited extent, the enclosing rocks may have quartz supplied to their inter- 

 stitial cavities. In the case, however, of the solid interior of a massive crystalline 

 rock, such change, by the substitution of quartz for some of the constituent minerals, 

 is so slow and so uncertain that it can be questioned whether it exist at all. No 

 evidence of it has been found in the course of the foregoing petrographical investi- 

 gation of the rocks of the state. When quartz is found to exist in any deep-seated 

 crystalline rock, it seems to be necessary to allow that it existed there from the date 

 of formation of the rock as such. It may be altered by metamorphic forces, as all the 

 minerals may be, and new crystalline conditions and even new chemical com- 

 binations may have been imposed upon it, but it seems necessary to exclude the idea 

 that changes of the relative amounts of the chemical elements have been effected. 

 The interior of the oldest rocks known are exactly as fresh and as complete as rocks 

 (excluding, of course, occasional abnormal conditions) as when they were first formed. 

 When a deep-seated rock is metamorphosed its elements are recrystallized in situ; at 

 least this is the case with the metamorphism of the Archean as exhibited in Minnesota. 



