l86 The American Geologist. Marcii, iguo 
within recent geologic time. The northern portion of this district, 
drained by the Monocacy and Antietam, seems never to have been re- 
duced to such a base-level. The reason for this difference is to be 
found in the fact that the course of the Mpnocacy lies over the hard 
quartzyte, while the Catoctin flows across the easily soluble granitic 
area. Owing to this difference in the underlying formation, the Catoc- 
tin has become the master stream of the valley. 
The Greater Appalachian valley, lying between the Blue ridge and 
the Alleghany front, admits of a two-fold division into the Great or 
Cumberland valley on the east and the Allegheny ridges on the west. 
The Great valley has a broad gently rolling floor, its former level 
surface being apparent in the coincident elevations of the present 
hills. The meandering of the streams, together with their frequent 
rapids, and the gorge-like character of their banks leads to 
the conclusion that their courses originated when the pres- 
ent valley floor stood at a lower level, the later elevation of the 
area causing these courses to be superimposed over their former flood 
plains, upon the limestone and shale beneath. Dr. Abbe finds two 
origins for the crests of the Alleghany ridges: — the higher ridges he 
finds due to the arching of resistant sandstones; the lower, to the de- 
pression of hard Carboniferous conglomerate below the level of the 
Schooley peneplain. This hard stratum thus failed to be removed in 
that base-levelling, and subsequent erosion has removed the softer 
layers overlying it. Thus the first class of ridges are anticlinal; the 
second, synclinal. During Mesozoic time the broad level floor of the 
Shenandoah plain was produced. The elevation of the plain is greater 
and its slope steeper than would be possible as the result only of 
the streams in connection with which it was formed. The explanation 
is to be found in a tilting along a northeast-southwest axis, after the 
plain was developed. This elevation had the effect of rejuvenating the 
Appalachian streams, causing them to cut deep channels below the 
surface of their flood plains. Three terraces along the banks of the 
Shenandoah streams show that the rate of elevation was not uniform. 
The Alleghany plateau presents the largest continuous remnant of 
the Schooley peneplain. It is dissected by two classes of streams: — 
subsequent and consequent. The subsequent streams are anticlinal and 
have cut deep valleys in the shales through which they have developed 
their courses. The elevation which terminated the formation of the 
Schooley peneplain, rejuvenated these streams. The consequent or 
synclinal streams have not cut through the hard conglomerate on 
which they originated, — hence their bounding slopes are gentler and 
their valleys stand at a higher level than those of the subsequent 
streams. 
Dr. Abbe concludes this portion of his work with a consideration of 
the general relations of the Alleghany streams to tlie structure of the 
region. He finds that although the Appalachian district was former- 
ly the coastal plain of the Piedmont, the majority of the streams flow 
eastward. The cause of this reversal of drainage is yet to be found. 
The results of the author's observations on the Piedmont drainage 
