114 Review of Recent Geological Literature. 
crystalline rocks, and the low coastal plain of slightly inclined Meso- 
zoic and Cenozoic formations. 
Notes of the rock exposures in more than sixty localities within the 
area of the detailed map illustrate very fully the variable characters of 
the formations represented, which include alluvium, the Columbia 
formation, the Sassafras river greensand, the Potomac formation, and 
the probably Archfean crystalline rocks. 
A remarkable feature of the entire tract is the general absence of 
subaerial alluvium. The streams northwest of the fall-line have high 
declivity and have cleanly swept their channels, while those on the 
southeast have deposited their detritus either in their own ever-widen- 
ing estuaries or in the bay. Subaqueous alluvial sands are penetrated 
to a depth of 140 feet by a boring at Fishing Battery station, with no in- 
dication of reaching the base of these postglacial deposits of the Sus- 
quehanna and its affluents. 
The loam or clay, sand, and gravel of the Columbia formation here 
represent, as stated by Mr. McGee, "a sub-estuarine delta of the Sus- 
quehanna river deposited when the Quaternary ice-sheet reached its 
southernmost extension, contemporaneous in a general way with the 
glacial deposits of the north ; and they indicate coeval but surprising- 
ly brief submergence of the region, reaching at least two hundred and 
forty feet and continuing sometime after the retreat of the ice-sheet 
commenced." According to the author's computation, the time occu- 
pied in the deposition of this ancient delta may not have exceeded 
1,500 years. It is referred to the earlier of the two principal glacial 
epochs of the Quaternary, which is believed to have been divided from 
the later glacial epoch by a longer time than that which has elapsed 
since the final disappearance of the ice. 
The Potomac formation, consisting of clays and sands of early Creta- 
ceous or late Jurassic age, stands next in the order of importance in 
this study, since the sandstone of its lower part would be the source of 
artesian water in this district. It is probable that this sandstone occurs 
beneath Fishing Battery station, but that the overlying clays of the 
upper part of the Potomac formation had been there eroded, allowing 
it to be directly overlaid by the less impervious Columbia formation. 
Therefore it seems doubtful, in view of the imperfection of the con- 
fining stratum and of the limited head, whether water reached in 
the Potomac sandstone would flow, though it would probably rise 
nearly or quite to the surface. 
Along the western border of the coastal plain and the fall line of the 
streams, before noticed, Mr. McGee finds evidence of displacement 
which has been in progress during the Quaternary and recent period, 
the area northwest of this line from the Rappahannock to the Hudson 
being uplifted, and that on the southeast being depressed. The sink- 
ing on the seaward side of the line appears to commence in the vicinity 
of Fredericksburg and to increase northward to 300 or 400 or possibly 
500 feet, while the area on the northwest has probably been elevated 
