MARYLAND GEOLOGICAL SURVEY 191

which occupy ancient depressions in the surface of the Patuxent for-
mation. These lenses have been traced all the way from Cecil county
to the border of the District of Columbia. The clays are highly
carbonaceous, lignitized trunks of trees being often encountered in an
upright position with their larger roots still intact. Scattered through
the tough, dark clays are vast quantities of nodules of iron carbonate,
at times reaching many tons in weight, and known to the miners under
the name of " white ore. " In the upper portion of the formation
the carbonate ores have changed to hydrous oxides of iron, which the
miners recognize under the name of " brown ore. " The largest lenses
have been found to reach a thickness of nearly 125 feet.

The fossils thus far found consist mainly of Dinosaurian remains,
which Professor Marsh regards as indisputable proof of the Jurassic
age of the deposits. Among the few plant fossils thus far collected
from this horizon no dicotyledonous types of vegetable life have as
yet been detected, and from what is known of the flora there is nothing
that would hinder its reference to the Jurassic. Both the physical and
paleontological characteristics of the deposits point to swamp condi-
tions as affording the only satisfactory explanation for the origin of
the formation. This could have been brought about by landward
tilting of the continent accompanied by a clogging of the drainage
lines.

THE CRETACEOUS PERIOD.

The formations grouped under this head comprise members of both
the lower and upper Cretaceous, a marked line of unconformity
occurring between the two groups. To the north of Maryland, in
eastern New Jersey, deposits of undoubted Cretaceous age pass con-
formably over into the basal strata of the Tertiary, but in Maryland
the break between the uppermost member of the Cretaceous series
and the Eocene is sharply defined. The Cretaceous deposits extend
as a broad belt from New Jersey across Maryland into Virginia. Five
distinct formations may be recognized in Maryland, viz.: the Patapsco
and Raritan formations of lower Cretaceous age and the Matawan,
Monmouth and Rancocas formations of upper Cretaceous age.

THE PATAPSCO FORMATION. —The Patapsco formation, so called