PART III. 



MINERALOGY AND PETROLOGY OF MINNESOTA. 



BY N. H. WINCHELL. 



(a) THE ROCK-FORMING MINERALS. 



(1) The White Minerals. 



Quartz. That mineral which is one of the most abundant, and which ordinarily 

 appears most refractory, being infusible and insoluble in ordinary conditions, is one 

 of the most mobile under the conditions that obtain in the rocky strata, and especially 

 under the forces that cause metamorphism. Its changes are visible even in the most 

 friable sandstones. When no other alteration is visible in some of the clastic rocks, 

 the quartz grains are seen to be enclosed in a new sheath of quartz oriented in the 

 same directions as the original grains. This has been shown by Irving and Van Hise 

 in the case of many of the rocks of the lake Superior region,* and had been noted 

 earlier by Sorby and by Bonney in the case of various European rocks. As this 

 change is the first symptom of metamorphism in any clastic rock containing quartz, 

 so it continues throughout the progress of metamorphism to be the most important 

 and the most active. It proceeds so far that the first clastic grains, whose forms are 

 wholly obliterated and which are entirely recrystallized, are those composed of 

 quartz, and, when the alteration of the rock is completed, the last mineral to have 

 been located in the new crystalline mass is uniformly quartz. It encloses all the 

 other minerals, or at least it occupies those spaces that were last filled. When the 

 feldspars are also metamorphosed, it sometimes penetrates them and fills the openings 

 between their cleavages. 



Quartz grains not only undergo enlargements by very slow and sometimes by 

 insignificant increments to their borders under the gentle influence of ordinary tem- 

 peratures, but they are powerfully affected when they come under the influence of 

 igneous contact. Here they can be seen to become bipyramidal and to take up the r61e 

 of phenocrysts in a quartz-porphyry (Nos. 619, 783, 784), while in the same rock mass, 

 at a short distance from the contact, the clastic structure of the quartz grains is 

 preserved (Nos. 264, 1838, 1839). 



*Bulletin via, U. S. Geol. Survey, 1884. 



