126 NATURAL SCIENCE. Feb.. 



gravelly and sandy tracts, the rainfall formed subterranean courses, 

 flowing, in some instances, even out to sea in that way, as we now 

 witness in the case of numerous springs issuing from our cliffs on the 

 coast. There deep channels or " chines " are in time formed, and the 

 gravelly accumulations being ultimately cleared out, the clayey strata 

 are exposed. In one case, I noticed a narrow gorge in the Contorted 

 Drift, 9 feet deep and 2^ to 3 feet wide — like a miniature canon — at 

 the bottom of one of these chines. Thus streams flowing for a time 

 underground, and carrying away material from below, will cause the 

 surface to sink. 



From these and other considerations we may be able to under- 

 stand how some of our great sheets of gravel and sand, like those of 

 the neighbourhood of Holt and Cromer, have become isolated hills, 

 with ramifying spurs ; for although some of the larger sheets of gravel 

 may have been deposited in patches, yet the surrounding strata have 

 been denuded, and they themselves have been broken up into 

 smaller patches or outliers. When once this has taken place, the 

 surface-features of these sandy and gravelly tracts may remain for 

 long periods much the same, although the level of the whole may be 

 slowly reduced by springs carrying away material from the lower 

 portions of the strata at their junction with clayey beds beneath (3). 



Subterranean erosion is suggested by the irregular channels that 

 are often met with on the surfaces of clays that lie beneath Valley 

 ■Gravel. On the Lias Clay at Bath, on the Oxford Clay at Oxford, 

 and on the London Clay in the Thames Valley, we find such 

 channels beneath the gravel ; and in many instances the stones 

 filling the channels lie with their longer axes more or less upright. 

 These appearances have sometimes been attributed to the action of 

 land-ice, or to the movements of thawing and slipping soil.^ Without 

 (questioning that these explanations may be true in certain cases, I 

 think the possibility of erosion by streams has often been over- 

 looked. 



On the Dorsetshire coast, between Lyme Regis and Bridport, 

 gullies similar to those noted on the Norfolk coast may be seen in 

 the Lias clays, and these are formed by springs that issue from the 

 porous Cretaceous Beds above. Conybeare, in 1S40, in his explana- 

 tion of the great Landslip of Dowlands and Bindon, remarked that 

 where the loose sands of the Upper Greensand rest on the Lias 

 clays, and are exposed in the cliffs, " copious land springs will gush 

 forth, and carry away in different seasons greater or less quantities of 

 the loose material through which they flow ; and thus, in process 

 of time, the superincumbent rock will become partially undermined." 



Some of the landslips on Portland, and the great gullies or 

 fissures that affect the limestone-rocks, may probably be attributed 



J See Letter of Darwin to Professor James Geikie (1876), Life and Letters of 

 Darwin, ed. 2, 1887, vol. iii., p. 214. 



