154 W. M. Fontaine— Mesozoic Strata of Virginia: 
reddish sand. From the character of the finer matter, and of 
its contained fragments, it is clear that much of it was derived 
from the Potsdam strata, on the west of the Blue Ridge. As 
T shall have frequent occasion to refer to this Potsdam material, 
a brief description of the occurrence and character of the parent 
rocks will be required. 
In my article on the “ Primordial Strata of Virginia,” pub- 
lished in the May and June numbers of this Journal for 1875, 
gave some account of certain varieties of the Potsdam strata, 
linearis in these erratics. This lower series is best ex at 
the town of Fredericksburg, where it, in its more coherent 
portions, yields a building stone, which was formerly used 
to 
he lower series passes up into a higher a of beds, con- 
ward. Near Alexandria, between Washington and Baltimore, 
and near the latter city, they constitute the whole of the uppe 
