DKAINAGE. 
that direction and composed of homogeneous material, had been exposed for a considei 
able time to erosion. It is known that the elevated peneplain slopes to the southeast, anoT 
the drainage of this area is therefore largely consequent — the result of the initial slope of 
the plain. But it has been shown that this plateau is composed of rock beds tipped on 
edge and striking northeasterly. These various beds of rock are of dissimilar composition, 
and hence possess different degrees of hardness and resistance to corrosion and corrasion. 
The natural result when streams flowing down a slope cross a discordant structure is 
adjustment to that structure, finally causing them to flow along the strike of the rocks. 
When such a result is not reached, as it is not in the present case, other conditions must 
have entered. 
The explanation which first suggests itself is that after planation the Piedmont Plateau 
was submerged, covered with a layer of sediments, and then elevated to its present stand. \ 
Consequent drainage established on its slope would cut through t-his mantle and be super- 
imposed on the transverse structure of the ancient rocks below. There is, however, no 
evidence preserved of such a submergence or of such a deposit, and this explanation may 
therefore be abandoned. 
A second explanation involves much the same principles as the first. It has been shown 
that the region now occupied by the Piedmont Plateau was eroded nearly to base-level. 
When this process, whose effect gradually lessened, had reduced the land well toward a 
peneplain, the rate of erosion became so slow that decomposition of the rocks — perhaps 
aided by the peculiar conditions existing at such a stage a — proceeded faster than degrada- 
tion. That decomposition has actually exceeded denudation is evidenced by the fact that 
many of the streams have not yet cut down to fresh rock; but the rate at which the streamj jT 
are now cutting, when compared with that in other regions, is almost certainly greater than ' 
that of decomposition — an indication that decomposition could have exceeded denudation 
only when conditions were different from those at present. 
During the interval from the time when decomposition began to proceed faster than its 
products could be carried away until the streams were finally rejuvenated by uplift, 
atmospheric alteration of the rocks had opportunity to extend far below the surface of the 
peneplain. It is probable that all the rocks of the region, including the quartzites (which, 
as will be seen later, are impure, holding much mica, and hence readily susceptible to dis- 
integration), were, at the surface where decomposition was greatest, reduced to soil. But, 
under the sluggish action of the streams, which had almost attained base-level, this soil 
remained in situ and still retained the structure of the rocks from % '.oh it had been derived. 
Here, then, was a layer of soft and relatively unresistant material lo serve as a mai^fe to 
the hard rocks below. In appearance it possessed a decided structure of its own; in 
essence it was at the surface almost structureless. 
The drainage consequent on the establishment of the present Piedmont slope began to 
trench this covering of soil and soon incised considerable valleys in it. As the streams 
wore down the beds of rock more resistant to weathering were discovered and eroded, 
but, in general, less rapidly than the surrounding beds. In this way the rapids and falls 
were formed. These barriers tended to divert the streams and adjust them to this.*new- 
found structure, but the valley walls already formed held the streams closely to>. their 
initial consequent courses. At present a number of streams have cut down to and into * 
solid, fresh rock, to whose structure they show very little adjustment. 
Such streams appear to belong in a subdivision of superimposed rivers. They have 
courses " quite out of accord with the undei 1) ing structures on which they have descended," & 
but the relatively thin overlying mass through which they have first cut, instead of being 
an unconformable deposit, as the usual definition of this class of rivers requires, is the 
irregularly decomposed zone of the deeper lying rocks. 
Other factors have doubtless helped to maintain this unadjusted condition of the streams. 
The principal one of these is steepness of slope to the southeast — i. e., across the structure. 
oCf. Campbell, M. R., Erosion at base-level: Bull. Geol. Soc. America, vol. 8, 1897, pp. 221-226. 
bDavis, W. M., Rivers of northern New Jersey: Nat. Geog. Mag., vol. 2, 1890, p. 82. 
