494 CLARK AND BIBBINS 
of marine Cretaceous. The section at this point is as fol- 
lows: 
ROCKY POINT, ROUND BAY, SEVERN RIVER. 
Matawan. Black laminated sands highly weathered, producing 
a reddish and grayish motted appearance to ft. 
Raritan. Coarse, gritty sands with lenses of clay; the sand 
cross-bedded and often indurated, forming heavy - 
ledges of ironstone 30 ft. 
Total thickness Ao ft. 
Other excellent sections are seen on the Magothy River, on 
the Lower Patapsco and its tributaries, on Elk Neck, and near 
the mouth of the Sassafras River, in which very much the same 
characters are exhibited. The light colored sands are espe- 
cially well developed on Elk Neck where they attain a very 
great thickness. 
Fossils —The fossils of the Raritan formation consist mainly 
of plant remains. Several brackish water molluscan shells have 
been obtained from the Raritan formation, farther to the north 
in New Jersey and it is not improbable that the same may be 
observed in Maryland when the deposits have been further 
studied. The fossil plants include dicotyledons of much more 
recent affinities than those of the Patapsco formation, the break 
between the two floras being very marked. 
INTERPRETATION OF THE DEPOSITS. 
The interpretation of the Potomac group involves the con- 
sideration of the sedimentation and structural relations of the 
deposits as well as of the entombed fossils, and each has an 
important bearing in the elucidation of the other. The con- 
ditions of sedimentation explain in no small degree the char- 
acter of the fauna and flora while their features likewise throw 
light upon the physical conditions which existed during Potomac 
time. 
Sedimentation and structural relations —The sedimentation of 
this period can best be understood after an examination of 
