THE ADIRONDACK GRAPHITE DEPOSITS 125 



of the change may be, the process furnishes some aid in the problem. 

 [f a large amount of pyroxene (say augite) is found in an amphi- 

 bolite, it suggests an igneous origin. Martin, 3 for example, points 

 out that the dynamic disturbances of the northwestern Adirondack's 

 (Canton sheet) were sufficient to all but completely transform the 

 pyroxene to amphibole. Thus the absence of augite does not prove 

 a sedimentary origin but may suggest it. On the other hand, the 

 presence of pyroxene points toward igneous origin. In neither case 

 is this criterion conclusive. 



Hunting for additional criteria, the writer investigated the feld- 

 spars in turn. Now Vogt, 2 Becke, 3 Marc, 4 and Harker 5 attempt 

 to present the physical chemistry of the system ; albite-orthoclase- 

 anorthite by a thermo-equilibrum diagram. The plagioclase series 

 is an isomorphous one*' (solid solutions), while the orthoclase-albite 

 pair is an eutectiferous one. 7 The third pair, orthoclase-anorthite 

 is very likely similar to the latter. Thus plotting the three com- 

 ponent system upon a triangular base, an eutectic line is to be drawn, 

 connecting the two eutectic points. If the feldspar composition in 

 the magma was on the potash side of this eutectic line, the resulting 

 crystals would approach the orthoclase type of feldspar, while if it 

 were on the other side plagioclase would result. But if the position 

 of the molten feldspar was on or near the eutectic line, the solid 

 minerals would be divided, on freezing, into orthoclase and plagio- 

 clase, usually in about equal amounts. In examining the slides of 

 the amphibolites, it was found that the undoubted sedimentary types 

 carried a motley collection of all sorts of feldspars with no definite 

 proportion among them, while the igneous varieties carried an 

 evenly split orthoclase-andesine content, for example. A rock with 

 both potash and soda-lime feldspars is termed a monzonite. While 

 it is not certain that the ferromagnesian minerals were original 

 pyroxenes, it seems likely, and hence the writer suggests that the 

 ortho-amphibolites he has encountered are in large measure meta- 

 augite-monzonites, although the name metagabbro is employed as a 

 more general term. 



1 Martin, J. C, X. Y. State Mus. Bui. 185, p. 57. 



2 Vogt, T- H. L., Silikatschemelzlosungen, 1914, 2:120-21. 



3 Becke," F., Tschermak, Min. u. Petro. Mitth. (2), 1906. 25:361, 383-85. 



4 Marc. Robert, Vorlesungen uber die Chemische Gleichewichtslehre, und 

 ihre Anwendung auf die Probleme der Mineralogie, Petrographie und Geologie, 

 fig. 68 and pages 69, 111-12. 



5 Harker, Alfred, " The Natural History of Igneous Rocks," 1909, p. 250. 



6 Day, Arthur L., & Allen, E. T., Carnegie Inst. Pub. 31. 



7 Warren, C. H., Proc. Am. Acad. Art and Sci., v. 51, no. 3, p. 127-54. 



