466 NEW YORK STATE MUSEUM 



lence of these small masses with the main one it is unsafe to say 

 that the latter is younger than the Grenville, though it is quite 

 likely. On the other hand if there are any rocks in the district 

 older than the Grenville they are here. 



The gabbro of the unmetamorphosed cores is exceedingly like the 

 gabbro found elsewhere cutting the anorthosite and syenite, and 

 regarded as the latest member of the general eruptive series. This 

 latter is often somewhat metamorphosed, but its metamorphosed 

 phases show about the same mineralogy as the unaffected rock, 

 though recrystallized into a granular rock, and do not run out into 

 amphibolites, so far as the writer's observation goes. The meta- 

 morphism of the one seems less profound and of a different type 

 from that of the other. The writer has never found these amphi- 

 bolitic gabbros in connection with the great intrusives, never ex- 

 cept in association with the granitic gneisses, the Grenville rocks 

 possibly excepted. The difference may perhaps be accounted for 

 on the supposition that the inclosing granitic gneisses were less 

 effective as a protecting buttress against the stresses producing 

 metamorphism, than were the massive and bulky anorthosites and 

 syenites. And while this may be true and the two gabbros, not- 

 withstanding their differences, be of the same age, it seems a much 

 less likely supposition than that the one gabbro is much older than 

 the other and its more profound metamorphism thus to be ac- 

 counted for. 



Within the limits of the quadrangle no satisfactory evidence re- 

 specting the relative ages of the two main constituents of the Long 

 lake gneiss, the granite and the gabbro, has been discovered. Else- 

 where in the Adirondacks however the writer has found amphi- 

 bolites, in all respects like those produced from the gabbro by meta- 

 morphism, distinctly cut by granites very similar to, if not identical 

 with, these granitic gneisses, indicating that the gabbro is older 

 than the granite. Since there is some question as to the precise 

 identity of each of the rocks concerned, it is not safe to theorize too 

 widely. It does however indicate the presence of a gabbro in the 

 region older than a granite, both of which have suffered intense 

 metamorphism ; and hence enforces caution respecting the tendency 

 to class all gabbros together because they are gabbros, and all 

 granites because they are granites. While in doubt regarding these 

 Long lake gabbros the writer is disposed to regard them as older 

 than the anorthosite, hence distinct from the later gabbro. 



