378 Mr. Lyell on the Boulder Formation, 



By landslips and slides. — The last remark leads naturally 

 to the consideration of every combination of causes which can 

 give rise to great disturbance in the overlying beds, while the 

 stratification of those below remains even and unchanged. 

 For striking examples of this pheenomenon the reader is 

 referred to figures 1 and 13, in which the superposition of 

 vertical to horizontal drift, and of huge fragments and needles 

 of chalk to horizontal chalk and crag, are clearly exhibited. 

 In order to explain these sections, we may imagine that banks 

 of mud and sand existed beneath the sea in which channels 

 were occasionally excavated by currents. In banks of this 

 kind off Great Yarmouth, a broad channel sixty- five feet deep 

 was found in 1836, where there had been only a depth of four 

 feet in 1822*. If the cliffs of loam or sand bounding this new 

 channel give way, large masses may descend bodily and as- 

 sume a vertical or curved position. They may easily escape 

 subsequent denudation, because the direction of the currents 

 are constantly shifting. Thus strata which have assumed a 

 vertical position may be forced laterally against the opposite 

 sides of the channels, where the beds have remained horizontal. 

 Both the juxtaposition of vertical and horizontal beds, and 

 the superposition of disturbed to undisturbed strata, may be 

 caused in this manner. The constant descent of strips of land 

 into river beds in the deltas of the Indus, Ganges, and Mis- 

 sissippi, on the subsiding of the annual inundations, are well 

 known, and may give rise to analogous effects. 



During the late landslip near Axmouth on the 24!th of 

 December 1839, a lateral movement took place, by which 

 masses of chalk and green sand, which had been undermined, 

 were forced more than forty feet in a seaward direction, and 

 thrown into great confusion, while the subjacent lias was not 

 disturbed t. The pressure moreover of the descending rocks 

 urged the neighbouring strata extending beneath the shingle 

 of the shore, by their state of unnatural condensation, to burst 

 upwards in a line parallel to the coast, by which means an ele- 

 vated ridge more than a mile in length, and rising more than 

 forty feet, has been made to form an extended reef in front of 

 the present range of cliffs. This ridge when it first rose was 

 covered by a confused assemblage of broken strata and im- 

 mense blocks of rock, invested with sea-weed and corallines, 

 and scattered over with shells, star-fish, and other productions 

 of the deep. 



* See Elements of Geol. p. 307- 



t 1 have been indebted to the kindness of the Rev. W.D. Conybeare for 

 a description and section of this landslip, which I have published in the 

 6th edition of the Princ. of Geol. vol. ii. p. 78. 



