DIFFERENTIATION OF THE SYENITE-GRANITE MAGMA 253 



Penokee-Gogebic district of Michigan and Wisconsin, Yan Hise and Leith 



say: 



"In general, in the district under discussion, the granites are of a somewhat 

 acidic type. However, in the central area besides the granites there are sye- 

 nites and even gabbros, and the three rocks seem to grade into each other." " 



Similar statements are made for the Yermilion district of Minnesota. 

 In both of these districts the Laurentian series shows variations much 

 like those of what the writer regards as a single syenite-granite intrusive 

 body in the Adirondacks. 



Even though syenite should in a few places be found cutting granite, 

 or granite cutting syenite, it would not necessarily prove the younger age 

 of the syenite in the one case or of the granite in the other, except in a 

 local and very restricted sense. Thus a granitic portion of the magma 

 may have consolidated or become very viscous, after which a still molten 

 portion of more syenitic composition was injected into the granite. Like- 

 wise it is by no means inconceivable that a solidified or very viscous sye- 

 nitic portion may have been intruded by a still molten portion of granitic 

 composition. Slight local differences in age would thus result, but, with- 

 out other proof, we should not be justified in concluding that the great 

 mass of granite is distinctly older than the great mass of syenite or vice 

 versa. This is an important consideration, so that the finding of an 

 occasional example of syenite cutting granite or vice versa would not 

 necessarily vitiate the conclusion tha"C the great bodies of Adirondack sye- 

 nite and granite are merely differentiation phases of the same great in- 

 trusive magma., 



Such an explanation of possible occurrences is, however, not needed to 

 help the writer out of a dilemma, because the surprising fact is that, in 

 spite of so many detailed observations by several workers, no definite case 

 of syenite cutting granite has been found and, so far as the writer is 

 aware, but one good case of granite cutting syenite is known. This latter 

 is Cushing's ''^Morris granite,^^ which, in small masses, cuts syenite in the 

 Long Lake quadrangle.^^ 



It is interesting to note that this very "Morris granite^^ illustrates the 

 principle just set forth, for, as Gushing says, it shows both a fine-grained 

 and a coarse-grained phase, with the coarse not only cut by the fine but 

 also grading into it. 



Similar cases of slight differences in age of the same intrusive magma 

 have been observed in connection with some of the later gabbros and dia- 



" Van Hise and Leith: U. S. Geological Survey Monograph 52, 1911, p. 226. 

 15 H. P. Gushing: N. Y. State Mus. Bull. 115, 1907, p. 482. 



XVIII— Bull. Geoi,. Soc. Am., Vol. 25, 1913 



