Ailing — Problems of Adirondack Precambrian. 61 



impossible to so classify the rock. In some doubtful 

 cases the rocks were studied petrographically. Speci- 

 mens were collected from rock masses where field 

 relations pointed to a definite origin. Microscopic 

 examination revealed striking similarities and a few dif- 

 ferences. The similarities need not be touched upon; it 

 is the latter that are important. If the rock is sedi- 

 mentary in origin and derived from a calcareous shale as 

 Cushing suggests, 23 quartz would be expected to occur, 

 as unmetamorphosed shales almost universally carry 

 some quartz. Thus if any original quartz is present in 

 an amphibolite it gives it a sedimentary look, for basic 

 (subsalic, femic) rocks are usually lacking in this min- 

 eral. On the other hand the absence of quartz suggests 

 an igneous origin, but this may not be a safe criterion, 

 in that quartz may have been reorganized into meta- and 

 trisilicates. 



Seeking for a more reliable distinction the pyroxene- 

 amphibole (the pyribole of Johannsen 24 ) content was 

 examined. . It is held by many geochemists 25 that pyrox- 

 ene is a high temperature mineral, while amphibole is a 

 lower temperature form. The change from one to the 

 other being a paramorphic (or "autometamorphic") 

 one — a change readily brought about by the stresses of 

 dynamic and static metamorphism, — the inversion of 

 pyroxene to amphibole furnishes some aid in the problem 

 in hand. If a large amount of pyroxene, such as augite, 

 is found in an amphibolite it suggests an igneous origin. 

 But under the stress of severe metamorphism this inver- 

 sion may be complete. Martin 26 found this to be true of 

 the amphibolite inclusions in the granitic rocks in the 

 Canton sheet (St. Lawrence Co.). Thus the absence of 

 augite does not necessarily prove a sedimentary parent- 

 age, but merely suggests it, This criterion, like the 

 former, is therefore regarded as inconclusive. 



Hunting for additional criteria, the writer investigated 

 the feldspars in turn. It was found that the igneous 

 types usually contained a simple range of feldspars, such 



23 Cushing, H. P. : N. Y. State Mus., Bull. 169, p. 19 and Bull. 191, p. 

 15, 1914. 



24 Johannsen, Albert: Jour. Geol., 19, p. 319, 1911. 



25 Elsden, J. V.: Principles of Chemical Geology 1910, p. 114. Becke: 

 Tsch. Min. u. Petr. Mitt., 16, pp. 327-336. Clarke, F. W.: U. S. Geol. 

 Surv., Bull. 616, p. 386. Lacroix, Mineralogie de la France, I, 1893-1895, 

 pp. 668-669. 



26 Martin, J. C. : N. Y. State Mus., Bull. 185, p. 157, 1916. 



