252 W. J. MILLER MAGMATIC DIFFERENTIATION AND ASSIMILATION 



Is an}' of the granite of the Adirondack region to be classed with the 

 so-called "Laurentian granite" of the Thousand Islands district, and re- 

 cently regarded by Gushing as older than the great mass of Adirondack 

 syenite ? He says : 



"Whether these Laurentian granites are recognizable, however, over any 

 considerable part of the Adirondack region, in distinction from granites of 

 later date, is a much less certain matter, though we believe it to be the case." " 



This is an important question which should be constantly kept in mind 

 in field work. So far as can be gathered from the descriptions of the 

 Thousand Islands granite and syenite, there appears to be no positive 

 e\ddence that the so-called Laurentian granite is actually older than the 

 syenite. In the detailed surveys by the writer of five or six quadrangles 

 scattered over the southern Adirondack region, areas of gray to red gran- 

 ites, much like the so-called Laurentian granite in composition and ap- 

 pearance, have frequently been met with. Compare, for example, the 

 analyses of the Laurentian granites of the Thousand Islands district with 

 the analyses of other acidic rocks from the syenite-granite series in the 

 above table. In not a single case has any positive evidence been forth- 

 coming to show that any of the Adirondack granites are distinctly older 

 than the syenite, while man}^ times the granites are clearly only acidic 

 variations or phases of the normal syenite. A few only of such definitely 

 proved variations have been given above. Of course this does not pre- 

 clude the possible presence of really older granites in these regions, be- 

 cause by no means all of the red granites demonstrably grade into syenite 

 on account of lack of proper exposures. In the face of tht fact, however, 

 that so much detailed work has been done by various geologists, it seems 

 little short of remarkable that if there are really two great intrusive 

 bodies — one of granite and the other of syenite — of distinctly different 

 ages, no proof has been obtained. If any such older granite does exist, i c 

 must be small in amount, and the writer believes it could rarely be suc- 

 cessfully mapped as a formation distinct from the granites, which are 

 certainly only phases of the syenite. 



In the light of present knowledge the writer believes that if the term 

 "Laurentian" is used at all in northern N'ew York, it should be applied 

 to the large bodies of syenite as well as granite for, with comparatively 

 slight possible exceptions, these rocks represent merely different phases 

 of what was a single great intruding magma. In this connection it is 

 interesting to note that, in their reference to the Laurentian series of the 



" H. p. Gushing : N. Y. State Mus. Bull. 145, 1910, p. 38. 



