408 
The American Geologist. 
June, 189G 
this and two laccolites are exposed in partial section. The horizons of 
intrusion are more than 600 feet below the base of the Dakota. The 
date of intrusion is approximately indicated as Eocene or late Cretaceous. 
The intruded rock is more basic than is ordinarily found in laccolites 
and is much more easily disintegrated by weathering. The neighboring 
sandstones and those portions of neighboring shales which have been 
baked by the intrusions resist erosion better than the igneous rocks, so 
that the laccolites find topographic expression in valleys instead of 
hills. A mass of altered sandstone, protecting a pedestal of shale and 
igneous rock, stands prominent above the country, constituting the crest 
of Twin butte, the most conspicuous landmark of the region. 
The rocks of the laccolite and dikes were described by Mr. Whitman 
Cross. The rock of the laccolite and of most of the dikes is a basic 
syenite porphyry, in which the ferro-magnesian silicates, augite, biotite 
and olivene, exceed the feldspathic constituent. Augite is the predom¬ 
inant silicate. These rocks are allied to a large series from the plains 
of Colorado and New Mexico, to be described by Mr. Cross. 
Mr. M. R. Campbell discussed the origin of the eastward facing 
scarp of the Blue ridge throughout North Carolina, which has been 
attributed (1) to the action of sea waves on an elevated coast, (2) to the 
normal erosion of a broadly uplifted peneplain, and (3) to deformation 
produced by radial movements in the crust of the earth. . The first and 
second theories he regarded as obsolete or insufficient. The third theory 
seems to offer the best explanation, but deformation alone could hardly 
produce the present scarp: there seems to have been modifying condi¬ 
tions which have not heretofore been formulated, but which were prob¬ 
ably the immediate cause of the scarp production. 
No radial movement is known to have occurred in the Appalachians 
of sufficient intensity to produce so steep a scarp, but if, during a period 
of baseleveling, a slow monoclinal uplift occurs, the portion of the 
region which is beyond the influence of the uplift will remain at base- 
level, whereas in that portion in which the movement is at a maximum 
the process of baseleveling will be interrupted, producing a very different 
succession of topographic forms. There will be an intermediate zone in 
which the forces of elevation and degradation will be balanced against 
each other. 
If the movement is relatively rapid, the peneplain will encroach but 
slightly upon the uplift. If the movement is slow the peneplain will 
encroach to a much greater extent not only along the streams, but in 
the inter-stream areas also. The result of this encroachment is to 
accentuate the slope produced by the uplift, and if the movement is ex¬ 
tremely slow the slope will become a scarp. 
If this hypothesis is correct the peneplain which caps the Blue ridge 
is continuous with the Piedmont plain at a very short distance from the 
foot of the ridge, but the intermediate, or sloping, portion of the old 
peneplain is almost completely removed by more recent erosion along 
the zone of tilting. 
In the vicinity of Roanoke, Va., this uplift turned toward the north 
and crossed the"Appalachian valley. In this portion of its course simi¬ 
lar results were produced, but the rocks are not hard enough to preserve 
the scarp as they do further south. In crossing the Appalachian valley 
this uplift protected the basin of New river against the encroachment 
of the Atlantic streams, which otherwise would, doubtless, have cap¬ 
tured its headwaters. In this region also some of the tilted portion of 
the older peneplain is probably preserved in an intermediate level which 
some observers have classed as a distinct peneplain. 
It seems probable that in other regions local uplifts have occurred 
during the continuance of periods of extensive baseleveling, and if so 
similar forms have probably been produced. W. F. Morsell. 
