24 THE HISTORY OF CREATION. 
sea, the deposited layers of mud get into continually deeper 
and quieter water, where they can become condensed into 
stone undisturbed. But when, on the other hand, the 
ground slowly rises, the newly-deposited layers of mud, 
which enclose the remains of plants and animals, again im- 
mediately come within the reach of the play of the waves, 
and are soon worn away by the force of the breakers, 
together with the organic remains which they enclose. For 
this simple but very important reason, therefore, abundant 
layers, in which organic remains are preserved, can only 
be deposited during a continuous sinking of the ground. 
When any two different formations or strata, lying one 
above the other, correspond with two different periods of de- 
pression, we must assume a long period of rising between 
them, of which period we know nothing, because no fossil 
remains of the then living animals and plants could be pre- 
served. It is evident, however, that these periods of 
elevation, which have passed without leaving any trace be- 
hind them, deserve a no less careful consideration than the 
greater or less alternating periods of depression, of whose 
organic population we can form an approximate idea from 
the strata containing petrifactions. Probably the former 
were not of shorter duration than the latter. 
From this alone it is apparent how imperfect our records 
must necessarily be, and all the more so since it can 
be theoretically proved that the variety of animal and 
vegetable life must have increased greatly during those very 
periods of elevation. For as new tracts of land are raised 
above the water, new islands are formed. Every new 
island, however, is a new centre of creation, because the 
animals and plants accidentally cast ashore there, find in 
