Gushing — Igneous Rocks of the Adirondack, Region. 291 



anorthosite in many places. The anorthosites themselves have 

 gabbro borders and are also cut by gabbro dikes. The four 

 eruptives seem all to be variants of the same magma, and not 

 to differ greatly in age. They are metamorphosed but the 

 metamorphism is not excessive, not so great as to destroy the 

 original structures and textures except locally. They are not 

 thoroughly granulated and gneissoid except in the extreme 

 east. Metamorphism increases in severity going east. 



There are also other eruptive masses, chiefly granite and 

 gabbro, perhaps some syenite, so far as known no anorthosite, 

 which are certainly older, far older, than the eruptives just 

 considered. The younger eruptives cut these out across the 

 strike, just as they do the Grenville rocks. They contain in- 

 clusions of them, just as they do of the Grenville rocks. My 

 note books contain several instances of granite-gneiss inclu- 

 sions in anorthosite, along the northwestern margin of the 

 anorthosite mass, the only portion of this margin which I 

 know in detail. Inclusions of granite-gneiss in the granitic 

 border phases of the syenite are also recorded, though of 

 necessity these are more difficult to distinguish than similar in- 

 clusions in the anorthosite. These eruptives are always more 

 heavily metamorphosed than are those of the later group. 

 The granites are completely granulated, are orthogneisses. 

 The gabbros are entirely changed to amphibolites. It is also 

 true, at least in the western Adirondacks, that the attack of 

 the old granites upon the inclusions which they hold, with the 

 production of soaked, and mixed, rocks, is much more promi- 

 nent than is that of the intrusives of the younger group upon 

 their inclusions. Though the only safe method of determining 

 the relative age of these intrusives is by their field relations to 

 one another, still each group has, at least in the western area, 

 a facies of its own which may be used with caution in making 

 an age determination. 



Granite Relations in the Thousand Island Region. — On 

 the New York side in the Thousand Island region both Smyth 

 and I have shown the presence of two granites of entirely dif- 

 ferent age, and we are in entire agreement as to their rela- 

 tions."* The correlation of the older of the two with the 

 Laurentian I must assume the responsibility for. This older 

 granite is an orthogneiss, which cuts the Grenville series. The 

 younger granite, Picton granite, cuts out both. Exposures are 

 extraordinarily abundant, and the field relations especially 

 clear. Fortunately also the present erosion plane cuts the 

 Picton granite on the very roof of the bathylith, so that it is 

 full of fragments of the older rocks in their original attitudes, 

 and the belts of Grenville quartzite and various schists, and 



*Bull. 145, N. Y. State Museum, pp. 41-43. 



