288 Gushing— Igneous Rocks of the Adirondack Region. 



Art. XVIII. — Age of the Igneous Rocks of the Adirondack 

 Region ; by H. P. Cushing. 



Introduction. — In discussing " Magmatic Differentiation and 

 Assimilation in the Adirondack Region " in a recent paper, 

 the author, Prof. W. J. Miller, mildly criticises certain views 

 which I have published concerning the age of some of the 

 Adirondack eruptives.* The general paper deals with problems 

 of differentiation and assimilation suggested by these eruptives, 

 matters concerning which there is, so far as I know, little 

 difference of opinion among those who have worked in the 

 Adirondacks ; in regard to which Dr. Miller is simply adding 

 his own testimony to the published results of those who have 

 preceded him there. But his work seems to have led him to 

 the belief that all the great eruptive bodies of the Adirondacks 

 are variants of a single period of intrusion, and are not of 

 widely different age. At least in the paper referred to he so 

 argues in regard to all the syenites and granites of the region. 

 My reference of certain granite-gneisses in the Thousand 

 Island region to the Laurentian does not meet with his 

 approval, and he refers to them as " so-called " Laurentian. 

 Lest silence on the part of those who disagree with his views 

 should be misinterpreted as giving assent to them, it seems to 

 me that a reply is necessary : not at all for the sake of being 

 controversial, but in order that the dissent may be made as 

 plain as possible. 



Geology of the Region. — The Adirondack highland consists 

 entirely of Precambrian rocks, which are rimmed by early 

 Paleozoics except for a short interval at the outlet of Lake 

 Ontario. But one Precambrian sedimentary series has been 

 recognized, the Grenville series. Of this but shreds and 

 patches are left, the whole having been torn to pieces by the 

 intrusions which have invaded them. So far as known, all the 

 igneous rocks of the district are younger than the Grrenville. 

 They consist of anorthosites, syenites, granites and gabbros. 

 All are somewhat metamorphosed and some are heavily so. 

 Metamorphism is more excessive on the east than on the west. 



Since all are younger than the Grrenville, and no other 

 Precambrian sedimentary formation is present, the age of 

 these eruptives can only be given in terms of their relations 

 to one another. 



Miller' 's Argument. — u With certain possible rather local 

 exceptions, the great masses of syenite and granite which are 

 the most abundant rocks in the Adirondack region, are regarded 



*Bull. Geol. Soc. Amer., vol. xxv, pp. 243-64. 



