INTRODUCTION. 3 
surface of the globe, we must be struck with the fact that the land 
seems all gathered about the North Pole. Indeed, geologists tell us 
that the heaving and uplifting force which elevated the northern 
continents above their overflowing seas is still in action, and that 
the shores of the northern lands are still rising. This fact is ascer- 
tained by observing and marking the level of the water in the 
fiords of Norway and on the shores of the Baltic. The conclusion 
has been reached that there is a rise in the land at the rate of four 
feet a century. At the same time the bed of the southern oceans 
must be ever sinking; for the coral polype cannot live, as we 
shall find, at a greater depth than 120 fathoms, and yet the ocean 
around the coral islands is of an enormous depth. What this 
upheaving force is, and why it should act only in the neighbourhood 
of the North Pole, is still a mystery. 
The line where land and water meet is the coast. Sometimes 
the continent pushes back the advancing waves with a rampart 
of rocks, sometimes boulders detached from the main cliff lie out 
in the sea breaking the force of the waves; these rocky and ser- 
rated coasts are found in Norway, Scotland, and other countries. 
The low coasts slope down with gentle declivities, and the land, 
as it were, glides beneath the surface of the water. These coasts 
are only found on the shores of countries whose soil is clay, or 
mud, or some post-pliocene deposit. 
Thrusting off from the land and sailing on the surface of the 
water, as we peer down into its clear bosom, we naturally ask, 
What is the depth of the ocean? The question is more difficult of 
solution than it at first appears. It would seem an easy task to 
let down a leaden weight attached to a line, and at once measure 
the depth; but when the line is perhaps two miles in length, the 
density of the water is suth that the lead is comparatively 
lighter, and has now much less inclination to sink than it had 
at the surface, so that it is easily carried away by the under- 
currents, and when the explorer of the ocean bottom fancies his 
sounding-lead is perpendicularly beneath him, it is perhaps half a 
mile distant, and thus the depth he registers is much greater than it 
ought to be. Laplace, from astronomical considerations, conciuded 
the mean depth of the ocean to be 1,600 fathoms. Humboldt 
gives his assent to this calculation. Dr. Young, another eminent 
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