chap, vi.] GEOGRAPHICAL AND GEOLOGICAL CHANGES. 
83 
Animals did not create the lime which they secrete from the sea- 
water, and therefore we have every reason to believe that the 
inorganic sources which originally supplied it still keep up that 
supply, though perhaps in diminished quantity. Again, the 
great lime-secreters — corals — work in water of moderate depth, 
that is, near land, while there is no proof whatever that there is 
any considerable accumulation of limestone at the bottom of the 
deep ocean. On the contrary, the fact ascertained by the 
Challenger , that beyond a certain depth the “ calcareous ” ooze 
ceases, and is replaced by red and grey clays, although the 
calcareous organisms still abound in the surface waters of the 
ocean, shows that the lime is dissolved again by the excess of 
carbonic acid usually found at great depths, and its accumula- 
tion thus prevented. As to the increase of limestones in recent 
as compared with older formations, it may be readily explained 
by two considerations : in the first place, the growth and de- 
velopment of the land in longer and more complex shore lines 
and the increase of sedimentary over volcanic formations may 
have offered more stations favourable to the growth of coral, 
while the solubility of limestone in rain-water renders the 
destruction of such rocks more rapid than that of sandstones 
and shales, and would thus lead to their comparative abundance 
in later as compared with earlier formations. 
However weak we may consider the above-quoted arguments 
against the permanence of oceans, the fact that these arguments 
are so confidently and authoritatively put forward, renders it 
advisable to show how many and what weighty considerations 
can be adduced to justify the opposite belief, which is now 
rapidly gaining ground among students of earth-history. 
Shore Deposits and Stratified Rocks . — If we go round the shores 
of any of our continents we shall alw r ays find a considerable 
belt of shallow water, meaning thereby water from 100 to 150 
fathoms deep. The distance from the coast line at which such 
depths are reached is seldom less than twenty miles, and is very 
frequently more than a hundred, while in some cases such shallow 
seas extend several hundred miles from existing continents. The 
great depth of a thousand fathoms is often reached at thirty 
miles from shore, but more frequently at about sixty or a hundred 
G 2 
