60 Ailing — Problems of Adirondack Precambrian. 



preted as belonging to the later group of plutonics. This 

 has an important bearing upon the amount of graphite 

 ore that could be expected in depth. If it proved to be 

 the Laurentian a greater continuation of the " Dixon' ' 

 schist could reasonably be expected. 



The later Metagabbro. 



An early metagabbro has already been mentioned as 

 occurring in the Adirondacks ; a pre-Laurentian-granite 

 eruptive. The writer feels confident that he has suffi- 

 cient proof to substantiate the claim that there is also a 

 pos£-Laurentian-granite metagabbro in this region. In 

 the southeastern Adirondacks W. J. Miller 22 has called 

 attention to the boss and pipe-like behavior of the later 

 gabbro. The behavior of this older rock is decidedly dif- 

 ferent both in its form and mineral make-up. It is often 

 laccolithic in character. The chief ferromagnesian min- 

 eral is hornblende which frequently becomes so dominant 

 that the rock should deserve the term hornblende ortho- 

 schist. The writer has evidence, he believes, that it is 

 later than the Laurentian granite, in an occurrence on 

 the Flake Graphite Company's land where the meta- 

 gabbro is cutting the Laurentian granite-injected Swede 

 Pond quartzite ; furthermore, the metagabbro is in turn 

 cut by the later granite. 



The finest example of this metagabbro known to the 

 writer is on the Hooper property, west of Whitehall, 

 where remnants of an extensive laccolith form a capping 

 rock to the Grenville sediments. In contact with the 

 Swede Pond quartzite the rock exhibits marked chill 

 effects which resemble in a remarkable way the normal 

 diabase of the region. It has lit-par-lit injected the sur- 

 face of the already soaked Swede Pond formation. At 

 one spot on the property a boss of the later granite cuts 

 the syntectic Swede Pond gneiss (the quartzite saturated 

 by magmatic solutions of the Laurentian), giving within 

 a limited area the age relations of the important rock 

 units of the Adirondacks. 



While there is no doubt in the writer's mind that this 

 metagabbro, as is shown here, is of igneous origin, the 

 distinction between many paramphibolites and ortho- 

 amphibolites is an exceedingly difficult matter. The 

 writer encountered several occurrences where it was 



"Miller, W. J.: N. Y. State Mus., Bull. 170, pp. 26-27, 1914. 



