A CONTRIBUTION TO THE GEOLOGY OF SOUTHERN MAINE 557 



i. The presence of alferric minerals, thus affecting the 

 distribution of alumina. 



2. The alferric mineral may be biotite, in which case there 

 will be less than normative orthoclase. 



3. The alferric mineral may be hornblende or augite of a 

 lime-bearing variety, in which case there will be less than 

 normative anorthite. 



4. The quartz content will depend upon whether 2 or 3 

 takes place, or if both upon the amount of biotite formed. 

 The silica in hornblende and augite is in approximately the 

 same ratios as in anorthite; biotite does not require as much 

 silica as orthoclase, hence more than normative free quartz 

 will be present in the biotite rocks. 



5. Zoisite may be present, affecting the distribution of 

 lime and alumina. 



6. Actinolite may be present, calling for a re-distribution 

 of lime and iron and magnesia from diopside. 



7. Normative hypersthene disappears under these re- 

 arrangements. 



Soda invariably remains unchanged, in albite. Quartz rarely 

 departs far from the estimated excess of silica. 



In addition to these chemical characteristics, there are certain 

 mechanical alterations, such as undulatory extinction, microcline 

 twinning, and granulation. 



A consideration of the minerals present and of the chemical 

 possibilities brings out the fact that the mineralogy is character- 

 istic of a zone of considerable depth. The minerals are those 

 of high specific gravity and are almost without exception those 

 that might be formed in igneous rocks under deep-seated condi- 

 tions. The garnet- staurolite-tourmaline group of minerals, of 

 still higher specific gravity and indicative of more intense or 

 longer continued metamorphism 1 are entirely absent. 



Reference should be made to the recent book of Grubenmann, 

 Die Kristallinen Schiefer. This book aims to classify the 

 metamorphic rocks on a basis primarily of chemical com- 

 position, and secondarily of the place where the metamorphism 



1 See Van Hise, Monograph XLVII, U. S. Geol. Survey, p. 183, et seq. 



