8 



SCIENCE. 



[Vol. XIX. No 465 



The evidence from New Jersey and^Pennsylvania goes to 

 show that after the tilting of the sandstones there came an 

 extensive period of denudation, which resulted in the pro- 

 duction of a more or less perfect plain, the so-called Creta- 

 ceous base-level, which can be seen in the level tops of the 

 New Jersey Highlands and of the ridges of Pennsylvania. 

 Following this came an elevation, giving the streams re- 

 newed energy, and resulting in the etching out of the softer 

 rocks down to another peneplain, the Tertiary base-level. 

 Finally another elevation gave the streams another period of 

 activity, and it is in this cycle that we find them to-day. 

 The larger streams, like the Delaware, have already sunk 



MY. 

 ^y^. 



their channels well into the Tertiary peneplain. It is with 

 some of the smaller ones that we have now to deal. 



Unless something had occurred to interfere with their 

 work in the previous cycle, which ended in the production 

 of the Tertiary peneplain, the streams of this district should 

 now be well adjusted to the structure. On examining the 

 map, however, we find that many of them show a tendency 

 to deflect downstream as they run towards the Delaware. 

 Such an arrangement is characteristic of the tributaries of 

 flood-plained master-streams, as is well shown in the case 

 of the Mississippi and the Po, and may perhaps be explained 

 in this case by the flood plaining of the Delaware during the 

 Tertiary period of base-levelling. Had such a flood-plaining 

 occurred before, i.e., during the Cretaceous base-levelling 

 epoch, the side streams would have already become adjusted 

 to the structure, for since Cretaceous time the whole surface 

 of the country has been worn down some hundreds of feet. 

 Flood-plaining such as that believed to have taken place 

 here, seems to be characteristic of large rivers during the last 

 stages of base-levelling, when, with a very gentle slope, they 

 build their deltas up-stream from their mouths, covering the 

 country on both sides with alluvium.' 



The flood-plaining of the Delaware would give the side- 

 streams a superimposed course on the Tertiary peneplain, and 

 as they cut down through the cover they would flnd them- 

 selves flowing across the outcropping edges of the underlying 

 strata of sandstone and shale. An arrangement of strata 

 such as that here presented gives an admirable field for the 

 adjustment of streams. It can be readily seen that if a side 

 stream works back along the strike of one of these beds, it 

 has, especially if the bed is soft, a much easier course than a 

 stream which has to cross the edges of many hard and soft 

 strata on its way to join the master. Perhaps this may be 

 more easily understood from the accompanying figure (Fig. 

 3), reduced from the contoured map of the Pennsylvania 

 Geological Survey, representing the district under considera- 

 tion. 



1 W. M. Davis : " The Geological Dates of Origin of Certain Topographic 

 Forms on the Atlantic Slope of the United States" (BuUetia Geol. Soc. of 

 America, Vol. 2, p. 530); " The Kiyers and Valleys of Penn." (Nat. Geog. Mag., 

 Vol. 1, No. 3); "The Geographic Development of Northern New Jersey " (Proc. 

 Boston Soc. Nat. Hist , XXIV., 1889). 



In this case Tohickon Creek, only the lower part of which 

 is shown, has its course directly across the strike of the beds 

 down to the Delaware, while Tinicum Creek goes along the 

 strike for some distance and thus has an easier course. The 

 result has been that a branch of the Tinicum has gnawed its 

 way back along the strike until it is now within less than half a 

 mile of the Tohickon. The Tohickon has a descent of some- 

 what over twenty feet in the first mile from this point, while 

 the branch of the Tinicum falls over eighty feet in the same 

 distance. The distance from the present divide to the Dela- 

 ware is about eight miles along the Tohickon, and about five 

 miles along the Tinicum. It is seen, then, what an advan- 

 tage the little branch of the Tinicum has over its larger rival. 

 The region where the contest is going on is just south of the 

 letter A in the figure, and as the more favored stream works 

 its way further and further back, the divide will be pushed 

 over the intervening space, and before long the Tohickon 

 will be captured and led out by a shorter and better course 

 through the Tinicum, leaving its lower part, beheaded, to 

 continue its way down the Tohickon valley. The region of 

 the divide is pretty level, being all enclosed by the 300 feet 

 contour, with a slight slope toward the Tohickon, and a 

 greater one toward the Tinicum, and if we get this idea of 

 migration clearly in mind, it seems almost as if we could see 

 the divide moving toward the Tohickon. There are few trees 

 to protect the surface there, and the crops of potatoes and 

 corn which cover the fields give a good opportunity for the 

 rain to carry away the soil. 



What is about to take place in the case of the Tohickon, 

 seems to have already happened further to the east. Here 

 again the Tinicum is the pirate. A glance at the figure will 

 make plain the state of the case. If the Tinicum is followed 



down its course to the Delaware it will be seen to make a 

 sharp turn to the north-east just at the point where its pirate 

 tributary comes in from the south-west. Knowing, as we do, 

 that the easier course lies along the strike of the beds and not 

 across it, we naturally turn to this point to see what has 

 taken place. If on coming down the Tinicum to this point 

 we continue to the south, we go for some distance up a small 

 stream flowing north, which comes down to the Tinicum 

 through a deep and rather narrow valley. Continuing our 

 walk along this creek, we soon come to a little sheltered nook, 

 where a picturesque farm-house stands, past which the creek 

 flows, coming in from the south-west. We now leave the 

 latter, and continue up a hollow to the south-east, and across 



