THE PROBLEM OF THE ANORTHOSITES 221 



that in one locality, in the vicinity of Long Lake, Cushing was able 

 to demonstrate that the syenite is younger than the anorthosite. 

 Yet even Cushing states: "The syenite and anorthosite seem surely 

 derivatives from the same parent magma and of no great difference 

 in age." 1 This aspect of the anorthosite, i.e., its intimate connec- 

 tion with the syenite, is emphasized in the area as a whole, where, 

 in spite of fairly good exposures, only one other locality showing 

 the intrusive relation of syenite to anorthosite has been found, but 

 where, on the other hand, types intermediate between the two are 

 rather commonly found. This feature of Adirondack igneous 

 geology has not been studied in detail except, apparently, at the 

 one locality in the Long Lake quadrangle, though it appears to 

 deserve such study since it marks the great similarity between the 

 Adirondack anorthosites and others, the Norwegian and Volhynian 

 occurrences, for example. In the writer's limited experience it was 

 found that the change from anorthosite to syenite was heralded by 

 the appearance of inclusions of potash feldspar in the plagioclase. 

 The inclusions are small patches, uniformly oriented and consti- 

 tuting therefore an antiperthite. 2 These inclusions often show a 

 rather peculiar feature which, so far as the writer is aware, has not 

 been noted elsewhere. Surrounding some of them and correspond- 

 ing in general though not in detail with the outline of the inclusion 

 is an area of plagioclase differing from the crystal as a whole. Its 

 outline is usually sufficiently sharp to make it possible to determine 

 that it has a slightly higher refractive index than the rest of the 

 plagioclase, besides a different position of extinction which makes 

 it a rather conspicuous feature. An extremely fine twinning, not 

 shown by the main body of the plagioclase crystal, can usually be 

 seen with high magnification. A suggested explanation of these 

 features is that the material of the microcline inclusions was origi- 

 nally in solid solution in the plagioclase and that on separating 

 from solid solution it left the plagioclase poorer in potash feldspar, 

 and therefore of higher refraction than the general body of the 

 crystal more remote from the inclusions. But the rims about the 

 inclusions usually have not much greater mass than the inclusions 



1 Bull. Geol. Soc. Am., XVIII (1907), 485. 



2 F. E. Suess, Jahrb. K. K. geol. Reichsanst., LIV (1904), 417-30. 



