tn the vicinity of Great Barrington, Mass. 363 
is probably Trenton; and, farther, the quartzite and the associ- 
ated mica schist (and gneiss), since they directly overlie the 
limestone, and besides are sometimes interstratified with it, are, 
with little doubt, either of the Trenton, or of the Cincinnati 
(Hudson River) group. Prof. Hitchcock makes the statement 
that the limestone of the western range is essentially pure car- 
bonate of lime, or contains very little magnesia if any, while the 
more eastern, both in Massachusetts and Vermont, is mostly 
dolomite. * 
the Quebec group; but differ in referring the other rocks 
{on the basis of facts since ascertained) to the Trenton and 
* How far this difference is connected with reget geological age has not 
to 
Adams and Williamstown, 
ie degen to Hitchcock 4 
@ eastern, contain little or no m sia. 
+ Amer. Phil. Soc., Jan. 1, 1841, This Journ., xlvii, 151, 1844. The Messrs. 
made the limestone Trenton, or ere: cheering ter taarniee tc: A 
: Potsdam, and the Taconic slates, with those west of them to the H 
Son River, Hudson River slates. 
