290 Gushing — Igneous Rocks of the Adirondack Region. 



same age. I do not know how he knows that. I personally 

 have a strong suspicion that both granites which I believe to 

 exist in the region have syenitic phases, though I cannot prove 

 it as yet. 



There is a lack of good cases of syenite cutting granite in 

 the Adirondack region, but there is a very good reason for 

 this lack. Much of the syenite magma was apparently very 

 hot, most of it shows marginal differentiation phases, and some 

 of it gives evidence of marginal assimilation, as in the case of 

 the syenite at Tupper Lake, which shows a basic border phase 

 adjoining anorthosite, and an acid border phase against granitic 

 gneisses.* In my experience much of the syenite of the 

 region shows a border phase of porphyritic granite, and the 

 chief dikes which run out from the syenite masses into the 

 surrounding rocks are of this porphyritic granite type. 

 This is especially true where the bordering rocks are older 

 granites, and accounts for the lack of syenite dikes cutting 

 them. But there are numerous instances known to me of the 

 one type of granite cutting the other. 



All large igneous intrusions are attended by the production 

 of dikes formed during the later stages of the intrusion, out 

 of the unsolidiiied magma residuum. It is a part of the business 

 of the field geologist to recognize such and to distinguish 

 between them and the other dikes formed at an entirely different 

 period. It is because I believe that I have successfully done 

 just that, that I am combating Dr. Miller's view. He seems 

 to fail to realize, if his contention be true, that there is a 

 considerable amount of granite in the Adirondacks which is 

 a variant of the syenite, and of substantially the same age, that 

 a good case of such a granite cutting an older granite is just as 

 significant as though normal syenite itself did the cutting. 

 It was precisely such a relation, magnificently exhibited, in the 

 Thousand Island region that led to my correlation. 



Time Relations of the Adirondack Erujptives. — In the 

 Adirondack highland there is a great group of eruptive bodies, 

 consisting of anorthosite, syenite, granite and gabbro. So far 

 as we know, the anorthosite is the oldest of the four, in this 

 related group. The only case so far recorded in which the 

 time relations between anorthosite and syenite have been 

 determined is in the Tupper Lake region, where it has been 

 shown that the syenite is the younger. The localities were 

 visited by the International Committee in 1907, and the relations 

 observed seemed conclusive to all members of the Committee. 

 Away from the anorthosite, on its opposite side, the syenite 

 develops a granitic border, and it is also cut by a similar 

 granite. Dikes of similar granite are also found cutting the 



*Bull. 115, N. Y. State Museum, p. 478. 



