24 NEW YORK STATE MUSEUM 



Most of the pegmatite, silexite and aplite by far occur within the 

 parent granites, though some also occur in the remnants of the old 

 rocks which were intruded by the granites. 



In many places the older rocks, including the Grenville series, 

 gabbro and hornblende gneisses (or amphibolites) , have been more 

 or less injected or assimilated by the Lyon Mountain granite and 

 its pegmatitic fades, giving rise to a great variety of mixed rocks. 

 Injection and assimilation of the old gabbro and hornblende gneisses 

 are of particular interest because, as a result of these processes, 

 the hornblende and hypersthene have usually been largely or wholly 

 transformed into green monoclinic (apparently mostly diallage or 

 diopsideaugite) relatively free from iron with accompanying separa- 

 tion or magnetite. Also in many places the Lyon Mountain granite 

 (or its pegmatite) is locally notably more basic than normal and 

 it then contains considerable green monoclinic pyroxene and magne- 

 tite, this change in composition having resulted from more or less 

 perfect assimilation of portions of gabbro or hornblende gneiss by 

 the granite magma. 



The magnetitic iron ore deposits of the quadrangle are of particular 

 interest, especially those in and near Lyon Mountain village where 

 mining operations are now being carried on by the Chateaugay Ore 

 and Iron Company. Many years ago iron ore was taken from about 

 a dozen other localities, mostly in the Saranac valley. Because of 

 the development of magnetite as a result of the transformation of 

 hornblende and hypersthene into monoclinic pyroxene, the injection 

 or assimilation rocks are commonly distinctly richer in magnetite 

 than either the normal Lyon Mountain granite or normal gabbro 

 or hornblende gneiss. But much of the derived magnetite was 

 carried away in magmatic solution and deposited elsewhere, in many 

 cases to form the ore zones. These iron ores are not, therefore, 

 magmatic differentiation deposits in the true sense of the word. 



With the possible exception of some gabbros later than the 

 granites, the only other rocks (not including the Pleistocene) of the 

 Lyon Mountain quadrangle are small dikes, mostly diabase, which 

 have been found in more than 120 localities. These dikes always 

 have sharp contacts, and they are the latest of the Precambrian 

 rocks of the region. 



Late Cambrian, and possibly also Ordovician, strata formerly 

 must have extended well into the quadrangle because the Potsdam 

 sandstone now lies close to its borders on the east and northeast, 

 and the Ordovician is not many miles away to the east. 



