SERPENTINE, 
91 
north and south. The external characters are very uniform 
through the whole distance, and it is very constantly associated 
with certain minerals. These have been already referred to. 
Certain minerals, too, are as constantly absent, as galena, the 
sulphurets of copper, iron, molybdena, and zinc. 
Notwithstanding the wide range of serpentine in this country, 
it occurs so rarely among the rocks of sedimentary origin that 
its age, even approximately, is left undetermined, in which 
respect it is in the same condition as the granites. In St. 
Lawrence and Jefferson counties, New York, the serpentine has 
evidently disturbed the Potsdam sandstone in numerous places, 
and it seems highly probable that both the serpentine and 
pyrocrystalline limestones were erupted subsequent to the com¬ 
mencement of the Silurian epoch. There is another instance 
of the occurrence of serpentine of a still later date: it is near 
the epoch of the consolidation of the waterlimes. The locality 
is in the vicinity of Syracuse, Onondaga county, New York. 
Here it is evidently a serpentine of contact, or a metamorphic 
serpentine. Those magnesian rocks are altered or changed 
into serpentine by proximity with some eruptive rock. The 
changes are variable. Some portions of the rock are perfect 
serpentines, passing into masses which are only slightly 
altered; and in a few cases the change has been still greater, 
as appears from the production of mica, forming a mass some¬ 
what similar to granite. These altered rocks are confined to a 
small area. Like granite, therefore, it appears that serpentine 
has been the product of different periods, and to have been the 
product of agencies which have operated in a manner similar 
to those which gave origin to granitic compounds. But it 
appears that it is possible only in a few instances to determine 
the time of its eruption, in consequence of its being so rarely 
connected in this country with rocks of a determined epoch. 
