DIFFERENTIATION OF THE SYENITE-GRANITEl MAGMA 249 



and by means of thin sections, a fine example of gradation from a basic 

 or gabbroic phase of the syenite to a pink granite. Beginning one-fonrth 

 of a mile northeast of the top of Mount Francisco, and passing a half 

 mile sonthwestward over the mountain top, one may observe the transi- 

 tion from a gabbroic rock through normal syenite and granitic Siyenite 

 into a pink granite. There is a progressive diminution of dark-colored 

 minerals from 19 per cent to 3I/2 per cent. From the normal syenite to 

 the granite the exposures are perfectly continuous, and to the basic sye- 

 nite they are nearly so. Evidently this is a clear case of magmatic differ- 

 entiation. 



Also in the Lake Pleasant quadrangle, Groff Mountain consists of a 

 mass of typical gray syenite porphyry which, toward the west, becomes 

 more quartzose, less porphyritic, and grades through granitic syenite into 

 pink granite, while toward the south it merel}'' loses its porphyritic text- 

 ure and passes into normal syenite. Thus we have here still other types 

 of differentiation illustrated. 



In the ISTorth Creek quadrangle the writer has frequently observed the 

 normal syenite passing by perfect gradations through granitic syenite to 

 gray or red granites on the one hand or granite porphyry on the other. 

 Most of these variations, at least, are very clearly only different phases of 

 the same great intrusive body. 



GENERAL CONSIDERATIONS 



In order to more clearly emphasize the great similarity in chemical 

 composition of the members of the syenite -granite series, the following 

 chemical analyses of typical members of the series from various portions 

 of the Adirondacks are here presented in tabular form : 



