Abstract
Trends of chemical variation show that most of the Adirondack amphibolites are of igneous origin and are not metasomatic nor sedimentary. Some para-amphibolites probably occur in addition, and these plot distinct, but more analyses are needed to define their trends of variation and thus prove their origin. As original local variation in the compositions of the mafic intrusions is a fact, it is possible that there was also original regional variation. This aspect must be considered in any study of variations in composition caused by metasomatism linked with metamorphism. It is possible to recognize and distinguish original variation and subsequent metasomatic variation in ortho-amphibolites. It is a bad practice to compare average analyses of ortho-amphibolites from one area with those of another in an attempt to discover metasomatic changes between two areas. These points must be considered in studying variations in mineral compositions with metamorphic grade.
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