264 ELEMENTARY GEOLOGY 
Perhaps a river mouth is shifting, so that upon a 
thick layer of sand, which has been built by wave 
action, a deep layer of mud is deposited where the 
river pours forth its sediment. Streams like the 
Yellow River of China not uncommonly shift their 
mouths over distances of many miles, each time in- 
troducing a change in the kind of material that is 
forming on the bottom. 
Or again, corals growing in clear, warm ocean 
water may become exterminated by a slight change 
in the conditions, introducing muddy or cold water. 
So a bed of limestone, constructed out of coral re- 
mains, may abruptly cease and be covered by an 
accumulation of clay. 
A third important cause is the change in the land 
level. In the later pages, we shall find that coasts are 
constantly changing as a result of movements of the 
land. ome shores are sinking, others are rising, and 
from these changes there are conditions introduced 
which now favor the deposit of conglomerate, now _ 
of sand, and later of clay or even limestone. 
A study of the rocks reveals infinite varieties of this 
nature. Layers of vegetable matter, which grew on the 
land, are buried beneath ocean sand or clay, showing 
that the coast was slightly lowered. In some sections 
of strata nearly the same conditions seem to have 
prevailed for long periods of time. In such cases 
