Natural History. 57 



the Trent ; and, as it deepened its bed, a " longitudinal " 

 valley began to form on the soft Keuper marls to the south, 

 where the Trent now flows. 



The river Idle, which runs into the Trent a little to the 

 north of Gainsborough and was then an independent stream 

 draining the land round Mansfield where it rises flowed down 

 this valley, deepening and widening it continually, till it 

 reached the Humber. Other brooks and rivulets, collecting 

 from the land around, helped on the work ; and, as this went 

 on year after year, and the valley was pushed back further to 

 the south, the Trent was reached at last, and tapped near 

 Newark ; perhaps, as Mr. Jukes-Brown suggests, " on the 

 occasion of some great flood, when the last intervening barrier 

 gave way." 



Whether this is correct or not and something of the kind 

 may have easily occurred I cannot doubt that, for a very long 

 time afterwards, the two opposing channels struggled for the 

 supremacy, and that the river flowed both ways ; but, as the 

 Humber continued deepening its bed, and, as a consequence, 

 deepening and pushing back its " longitudinal " valley also, 

 while the land between Newark and Lincoln, where the old 

 channel ran, was reduced almost to a level, the result was 

 inevitable ; the captured Trent gave way at last to the yielding 

 marls of the Keuper, and no longer a " primary " but a 

 " subsequent " stream became a tributary of the Humber. 



A reference to the accompanying diagram will tend to make 

 this more clear. 



This is but a mere outline of the subject, and those who 

 wish to know more of it should read Mr. Jukes-Browne's 

 paper ; and also an article in the magazine of the Royal 

 Geographical Society, February, 1895, by William Morris 

 Davis, Professor of Physical Geography, Harvard University, 

 u On the Development of certain English Rivers," which deals 

 with the subjecl: more fully and elaborately than has been 

 attempted by any previous author. A careful perusal of this 

 most able and instructive paper will well repay the reader for 

 his trouble ; and, at the risk of wearying you with somewhat 

 technical details, the following short summary of the views put 

 forth in it will not, I think, be out of place. 



Prof. Davis begins his paper with the following thesis : 



"The rivers of Eastern England have been developed, in 

 their present course, by the spontaneous growth of drainage 

 lines on an original gently inclined plane, composed of 



