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University of California Publications. [Geology 



chemical properties of the species, as the equally important fam- 

 ilies of the feldspars and pyroxenes. 



This comes naturally from the complicated chemical consti- 

 tution, and from the difficulty of bringing into close and con- 

 tinuous relation the physical properties and the chemical compo- 

 sition of these minerals. Yet, when we try to introduce order 

 into this group, taking as our basis either the chemical composi- 

 tion or the optical or other physical pi'operties (e.g., etch-figures, 

 etc.), we find ourselves confronted by very many species, sub- 

 species, and varieties, where often the different individuals stud- 

 ied are variable in their composition and properties, forming 

 intermediate terms not easy to classify. 



Isolated mineralogical investigations of different members of 

 this family, whether well crystallized or not, occurring fre- 

 quently in small quantities, differing from each other in locality, 

 in occurrence, and in genesis, will hardly give the possibility of 

 a natural classification. We should much more easily attain re- 

 sults by an investigation of these minerals when they occur in a 

 series of rocks from a petrological province. Following the 

 changes in the mineralogical and chemical composition of the 

 rocks, we can learn the variation which the minerals themselves 

 undergo, and perceive the direction of the variation in their 

 chemical composition and physical properties. Such a study must 

 include, besides a precise and detailed determination of the phys- 

 ical properties, also a series of exact chemical analyses of the 

 individuals studied. 



During a visit to America I had the opportunity of studying 

 two important series of rocks with amphiboles : rocks with rie- 

 beckite (from Massachusetts, Dobrogea, etc.) ; and rocks with 

 glaucophane (from California). These two series comprise large 

 numbers of rocks with various amphiboles from hornblende to 

 riebeckite, or from actinolite to crocidolite. In these investiga- 

 tions defects which the classification and nomenclature of the 

 amphiboles present become evident, yet I was fortunate enough 

 to find some facts which throw light on the whole question. It 

 has become clear that the determination of even the most common 

 amphibole must not be limited merely to the measurement of the 

 extinction angle and pleochroism. 



