
Part IL. Secor. ii. §2.)  LANDSLIPS. ee sy, 
with volume enough to be navigable almost up to their outflow. 
In such circumstances lakes, either temporary, like the Lake 
Zirknitz in Carniola, or perennial, may be formed over the sites 
of the broken-in caverns; and valleys may thus be deepened, or 
perhaps even formed. Mud, sand, and gravel, with the remains of 
_ plants and animals, are swept below ground, and sometimes accumu- 
late in deposits of loam and breccia so often found in ossiferous 
caverns (Figs. 101 and 102). 
II. Mechanical Action.—In its passage along fissures and 
channels, underground water not merely dissolves and removes 
materials in solution, it likewise loosens finer particles and carries 
them along in mechanical suspension. This removal of material 
sometimes produces remarkable surface changes along the side of 
steep slopes or cliffs. A thin porous layer, such as loose sand or ill- 
compacted sandstone, lying between more impervious rocks, such as 
masses of clay or limestone, and sloping down from higher ground, 
so as to come out to the surface near the base of a line of abrupt cliff, 
serves as a channel for underground water which issues in springs or 
in a more general oozing at the foot of the declivity. Under these 
circumstances the support of the overlying mass of rock is apt to be 
loosened; for the water not only removes piecemeal the sandy 
layer on which that overlying mass rests, but as it were lubricates 
the rock underneath. Consequently at intervals portions of the 
upper rock break off and slide down into the valley or plain below. 
Such dislocations are known as landslips. 
Along sea-coasts and river valleys, at the base of cliffs subject to 
continual or frequent removal of material by running water, the 
phenomena of landslips are best seen. The coast line of the British 
islands abounds with instructive examples. On the shores of Dorset- 
shire, for instance (Fig. 104), impervious Liassic clays (a) are over- 

Fic. 104.—Srotion or Lanpsiip Frormine UNDERCLIFF, Prnnay, LymE-Reeis (B.). 
laid by porous greensand (6), above which lies chalk (¢) capped with 
gravel (d). In consequence of the percolation of water through 
the sandy zone (6) the support of the overlying mass is destroyed, 
and hence from time to time segments are launched down towards 
the sea. In this way a confused medley of mounds and hollows (/) 
forms a characteristic strip of ground termed the “ Undercliff” on 
this and other parts of the English coasts. This recession of the 
upper or inland cliff through the operation of springs is here more. 
