72 NEW YORK STATE MUSEUM 



is less abundant than is the case at Graphite and Hague. There is 

 no question but that it represents the same stratigraphic unit. 

 Beneath the Hague is the para-amphibolite, already mentioned, the 

 dresden amphibolite. In composition, character and in habit it is 

 similar to the Beech Mountain amphibolite, but this occupies a totally 

 different and distinct horizon. 



Two igneous rocks, both later than the Grenville rocks and which 

 have an important bearing upon the extent of the ore, are shown. 

 The first one to be noted is the black, gabbroic rock that acts as a cap 

 to the long ridge running east and west. This is the so-called 

 Laurentian metagabbro (formerly an augite monzonite) that occurs 

 as a big sill or thin laccolith covering and replacing in part the 

 syntectic Swede Pond gneiss. It formerly spread over a much more 

 extensive area than is shown today, for erosion has removed large 

 quantities, only small patches remaining. Even in the vicinity of the 

 mill small " skins " of the metagabbro adhere to the Swede Pond 

 gneiss. It was chilled rapidly on contact with the country rock, and 

 specimens studied microscopically show a very fine-grained rock 

 with diabasic texture but comparatively free from ferromagnesian 

 minerals. To the west this cap of the metagabbro keeps cutting out 

 more and more of the Swede Pond gneiss until it actually cuts the 

 ore itself. This marks the westward extent of the available ore, the 

 eastern end of the outcrop being obscured by a swamp. No one 

 can tell where the feeding channels of the metagabbro are, nor 

 whether they will be encountered in extensive underground 

 operations. 



One of the nice problems of Adirondack geology is the origin of 

 the amphibolite. As has been pointed out before (in the intro- 

 duction to the graphite deposits of the southern area) an " amphi- 

 bolite " may be of one of three origins. The metagabbro of the 

 Hooper property is- truly igneous. The criteria used in dis- 

 tinguishing it from the other types are given in the summary. It 

 remains to discuss its age relations. It lies on top of and has " lit- 

 par-lit " injected the syntectic Swede Pond gneiss. This gneiss was 

 previously saturated and injected by the Laurentian granite. Thus 

 one is forced to conclude, contrary to Cushing's experience 1 that it is 

 later in age than the Laurentian granite. Furthermore, it was found 

 to have been folded with Grenville rocks before the intrusion of the 

 later granite which is referred to the Algoman. It would be 

 stretching the matter beyond the known data to conclude that all 



1 Personally communicated. 



