292 Gushing — Igneous Hocks of the Adirondack Region. 



the orthogneisses as well, can be mapped right across the Pic- 

 ton granite surface just as accurately as though the granite 

 were not there. The relations show clearly and unmistakably 

 that the orthogneisses are much more closely related to the 

 Grenville in age than they are to the Picton granite. The 

 Picton is but slightly metamorphosed. Its margins and its 

 dikes are fine-grained, and are difficult to distinguish from the 

 orthogneiss, but as to the main mass the difference is marked. 

 In the vicinity there is a small stock of syenite, which I called 

 the Alexandria syenite, which has a border of porphyritic 

 granite, and this also contains inclusions of orthogneiss. 



The orthogneiss contains everywhere abundant inclusions of 

 amphibolite, and inclusions of little else. It also is sometimes 

 involved with larger masses of amphibolite, which it cuts. 

 Some of these certainly represent ancient gabbros or basalts 

 and are older than the orthogneiss. 



I regard the Picton granite and the Alexandria syenite as 

 local representatives of the great igneous bodies of the Adiron- 

 dack region, the anorthosite-syenite group, and as of similar 

 age. The orthogneiss is certainly an eruptive, a granite, and 

 as certainly greatly older. In the table of analyses which he 

 publishes, Dr. Miller lists two analyses of these orthogneisses, 

 calling attention to their similarity with the syenites farther 

 east.* It is greatly to be regretted that he omitted the analysis 

 of the Picton granite from this table. Anyone willing to take 

 the trouble to look up this analysis and compare it with those 

 of Miller's table, especially with JSTos. 6 and 7, and at the same 

 time reflect on the demonstrable age difference between the 

 two granites near Alexandria Bay, will be in excellent position 

 to appreciate how little bearing chemical similarity may have 

 on age.f 



Age of the Orthogneiss. — Having thus established to my 

 satisfaction the occurrence of two granites of widely different 

 age, and this in that part of the New York Precambrian which 

 directly connects with the Canadian Precambrian across the 

 river, I naturally cast about to see if any correlation could be 

 legitimately suggested. The Picton granite could not be 

 found cutting the Alexandria syenite, but I had little question 

 about correlating them both with the general eruptive series of 

 the Adirondacks. Then came the correlation of the orthogneiss. 

 The geologists working in Ontario and in the Lake Superior 

 region have shown that there are at least two granitic invasions 

 there, one cutting the Keewatin, but older than the next 

 younger series of sediments (Lower Huronian, Temiskaming, 

 Sudbury), and the other cutting through that series. Though 



*Op. cit., p. 250, analyses 9 and 11. 



f Bull. 145, N. Y. State Museum, p. 176, analysis 6. 



