SIGNIFICANCE OF THE VARIABLE COMPOSITION 411 



single mineral plagioclase, must have been at an exceptionally high tem- 

 perature if it was ever molten as such, but that anorthosite gives no evi- 

 dence of ever having been hot enough to melt plagioclase. Bearing on 

 this whole problem of the temperature relations of the anorthosite, we 

 should keep in mind the following statements by Clarke :-° 



"Melting points are modified by various agencies within the eartli, and it is 

 not yet possible to strilve a definite balance between the opposing forces. . . . 

 I'ressure tends to prevent the escape of dissolved vapors, and so to increase 

 fluidity. ... In the geological interpretation of the melting points (of 

 minerals) there is one particularly dangerous source of error. We must not 

 assume that the temperature at which a given oxide or silicate melts is the 

 temperature at which a mineral of the same composition can crystallize from 

 a magma." 



Much yet remains to be learned about temperature relations of molten 

 minerals and rocks within the earth, and we should, therefore, hesitate, on 

 the basis of experiments on simple melts of practically pure minerals in 

 the laboratory, to come to anything like definite conclusions regarding 

 the temperature relations of magmas under deep-seated geologic condi- 

 tions. 



Another important consideration is the almost certain presence of very 

 appreciable amounts of dissolved vapors, particularly water vapor, in the 

 magma ; for, as Clarke says, "one effect of the water would be to reduce 

 the temperature at Avhich liquidity could be maintained.^' Also the pres- 

 ence of about 2 per cent iron oxide in the typical anorthosite should not 

 be overlooked, for it is well known that increase in the iron content lowers 

 the melting point. 



All things con^sidered, therefore, I not only think it very reasonable to 

 apply the mutual solution theory to the anorthosite, but also to regard the 

 anorthosite to have existed in magmatic condition at a moderate tem- 

 perature. 



Since the foliation of the anorthosite is essentially a magmatic flow- 

 structure, I believe it proves that, at the very least, large portions of the 

 great mass of the anorthosite once possessed enough fluidity to permit 

 distinct magmatic currents or movements. The significance of the folia- 

 tion is not touched on by either Bowen or Cushing, but it is an important 

 consideration. For a full discussion of flow-structure foliation in the 

 Adirondack intrusives the reader is referred to my recent paper.-^ Only 

 the salient points pertaining to the anorthosite are here presented. Even 

 the typical Marcy anorthosite, almost entirely free from femic minerals, 



20 F. W. Clarke : U. S. Geol. Sur. Bull. 391, pp. 277 and 281. 



21 W. J. Miller : Jour. Geol., vol. 24, 1916, pp. GOO-619. 



