RIVER VALLEYS. 403 



To gain a general idea of the nature, extent, and 

 value of his scientific work, it will be convenient to 

 subdivide his writings according to the several branches 

 of geology which they illustrate. For this purpose we 

 may first consider his contributions to our knowledge 

 of the causes that produce geological changes, and the 

 effects to which they give rise. 



From an early part of his scientific studies Prestwich 

 paid close attention to the influence of running water 

 on the face of the land. His interest in this subject 

 was greatly quickened by his observations in connection 

 with the high-level and low-level Gravels of the river- 

 valleys in the south-east of England and the north-east 

 of France. From these deposits he drew the important 

 conclusion that the valleys have been mainly eroded by 

 the rivers which still flow in them. Though this ex- 

 planation of river- valleys was strongly insisted upon by 

 Hutton and Playfair, and had been demonstrated for 

 Central France by Desmarest and afterwards by Scrope 

 and Lyell, it had never attained wide acceptance 

 among geologists. When it was adopted and enforced 

 by Prestwich on a basis of well -ascertained fact, it 

 came almost with the freshness of a new discovery. 

 He quickly saw its significance in regard to the slow 

 sculpture of the face of the land, and the great antiq- 

 uity which it proved for the older and higher terraces 

 of Gravel. In his memoir, read before the Royal 

 Society in 1862 [56], 1 he dwelt on the evidence that 

 could be adduced of powerful and long-continued erosion 

 in the valleys by the streams that still flow in them ; 

 and he continued to bring forward additional proofs in 

 support of his views [61], until geologists everywhere 



1 The numbers within square brackets refer to the corresponding entries 

 in the list of writings given at p. 422. 



