72 
GILBERT. 
If, on the other hand, there was no initial contrast in 
density between range and trough and their relation is sus¬ 
tained by rigidity, there should be an excess of matter under 
the ridges and a defect, either relative or absolute, under the 
troughs. The looser aggregation of the detrital material in 
the troughs should make the actual contrast of attraction 
somewhat greater than would be estimated from the config¬ 
uration alone. Under this assumption the mean plain of 
reference for any station should include ridge and valley 
alike, and high stations should in general show, after reduc¬ 
tion to mean plain, a stronger attraction than low stations. 
Including in the reduction so much of the Wasatch and 
Oquirrh ranges as lie within 30 miles of Salt Lake City, we 
obtain an excess of 900 rock-feet ( b , Table III). This points 
toward a general excess of gravity for the province, but 
little reliance can be placed on the indication of a single 
station. 
One station only, Deer Park, belongs to the Appalachian 
mountains. Once the scene of pronounced corrugation, the 
Appalachian belt has been base-leveled, then raised in broad, 
flat arches, and finally dissected by streams. In the lati¬ 
tude of Deer Park its average altitude above neighboring 
plains is between 1,000 and 1,500 feet, a difference accord¬ 
ant with its excess of attraction, 1,300 rock-feet. 
The Piedmont plain has been long exempt from corruga¬ 
tion, was gently lifted in connection with the arching of the 
Appalachian belt, and has been degraded almost to the same 
extent. Theoretically, an indication of equilibrium should 
be anticipated. Charlottesville and Princeton give nominal 
excesses, 200 and 100 rock-feet. Boston, which is doubtfully 
assigned to this province, gives an excess of 900 rock-feet, 
an anomaly for which no adequate explanation has been 
suggested. 
Washington, with an excess of 1,600 rock-feet, and Phila¬ 
delphia, with an excess of 1,300, stand at the “ fall-line ” 
which divides the Piedmont and coastal plains. West of 
this line there has been much degradation in Cenozoic and 
