378 Mr. Lyell 07 i 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 phaenomenon 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, w^here 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 f. 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. 
