124 NEW YORK STATE MUSEUM 



striking similarities and a few differences. The similarities need 

 not be touched upon. It is the latter that are important. 



The interpretation of para-schists and para-gneisses should be 

 guided by mental reference to the original unmetamorphosed rock. 

 What kind of a sediment did we have in the beginning? Cushing 

 suggests a calcareous shale. 1 Now as the first point in our examina- 

 tion it is difficult to conceive of a shale without any free quartz. 

 If any original quartz is present in an amphibolite, it gives it a 

 sedimentary look, for basic (femic) rocks are usually lacking in 

 this mineral. But, on the other hand, the absence of quartz does 

 not furnish a reliable criterion for igneous origin, for recrystalliza- 

 tion may have brought about the formation of various silicates, 

 • using up the quartz present. 



The pyroxene-amphibole (the " pyribole " of Johannsen 2 ) con- 

 tent was next examined. It is held by many geochemists that 

 pyroxene is a high temperature mineral, while amphibole is a lower 

 temperature form, 3 the change from pyroxene to amphibole being 

 a paramorphic (or " autometamorphic ") one. Whatever the nature 



Chemical analysis of para-amphibolite (hornblende schist) from Lead hill* 



Ticonderoga 



Si0 2 



Al 2 Os •• 



Fe 2 3 



FeO 



MgO 



CaO 



Na 2 



K 2 



H 2 0— 



H 2 + : 



Ti0 2 



C0 2 



P 2 5 



S 



MnO 



FeS 2 



C • .■ None 



48 



26 



13 



32 



I 



4i 



II 



55 



6 



66 



10 



55 



3 



36 







80 





06 





96 



1 



99 





3« 





20 





20 





14 



99-84 



Less O .10 



99-74 

 Collected by E. S. Bastin, analyzed by George Steiger in the laboratories of the 

 United States Geological Survey, U. S. G. S. Bui. 591, p. 40. 



1 X. Y. State Mus. Bui. 169, p. 19, and Bui. 191, p. 15. 



2 Tohannsen, Albert, Jour. Geol. 191 1, 19:319. 



3 Elsden, I. V., " Principles of Chemical Geology," 1910, p. 114. 

 Becke, F., Tschermak, Min. u. Petro. Mitth. 16:327-36. 

 Clarke, F. W., U. S. G. S. Bui. 616, p. 386. 



Lacroix, Mineralogie de la France. 1893-95, 1:668-69. 



