420 W. MM. Fontaine— Primordial Strata of Virginia. 
from the oxidation of the minute amount of iron present. c¢ 1s 
almost always present in some portion of the Potsdam strata, 
and may be considered in this part of Virginia to be character- 
istic of this formation. So clearly marked are the features of 
this material that a fragment of it may be recognized at a 
glance wherever found. 6 and c are, when fresh, very light 
of water and lime. The rock, ¢, contains also a trace of iron, 
and a little mica in fine scales. The conglomerate near the 
ase of the system does not contain crystalline feldspar, 
because of the greater metamorphic action to which it has 
been subjected. The finding of feldspar in all stages of decay 
shows that this mineral is undergoing a process just the 
reverse of regeneration, and that strata composed of the fresh 
particles of degraded crystalline rocks have been mistaken for 
rocks partialiy crystallized by metamorphic action. 
The beds last described have a i aces of 300 feet, and 
are succeeded by a partially-concealed interval, in which 50 feet 
of a similar rock are shown. From this point, for the space of 
half a mile no cuttings exist.* The space outside of the valley 
in which the road runs is occupied by low rounded bills, cov- 
ered to a great depth by fragments of the rock last described. 
So great is this mass of matter that in some places where it 18 
removed to be used as ballast for the railroad, excavations to 
the depth of thirty feet do not penetrate through it. This inter- 
val up to the valley of the South Branch of the Shenandoah, so 
far as can be judged from the partial exposures, is occupied by 
No. (7). Much of the loose matter on the surface seems to have 
* No reliable estimate of the thickness of (7) can be given. It is perhaps 
800-1000 feet. 
