CHAP. X.] CONTINUITY OF THE CHALK. 477 
The wide extent of Tertiaries in Europe and the 
north of Africa sufficiently proves that much dry land 
has been gained in tertiary and post-tertiary times, 
and the great mountain-masses of Southern Europe 
give evidence of great local disturbance. But al- 
though the Alps and the Pyrenees are of sufficient 
magnitude to make a deep impression upon the 
senses of men, taking them together, these moun- 
tains would if spread out only cover the surface 
of the North Atlantic to the depth of six feet, and 
it would take at least two thousand times as much 
to fill up its bed. It would seem by no means im- 
probable, that while the edges of what we call the 
great Atlantic depression have been gradually raised, 
the central portion may have acquired an equivalent 
increase in depth; but it seems most unlikely that 
while the main features of the contour of the northern 
hemisphere remain the same, an area of so vast extent 
should have been depressed by more than the height 
of Mont Blane. On these physical grounds alone we 
are inclined to believe that a considerable portion of 
this area has been continually under water, and that 
consequently a deposit has been forming there unin- 
terruptedly, from the period of the chalk to our own. 
I will now turn to the paleontological bearings of 
the question. Long ago Mr. Lonsdale showed that 
the white chalk was mainly made up of the débris of 
foraminifera, and Dr. Mantell estimates the number of 
these shells at more than a million to a cubic inch. 
In 1848, Dr. Mantell, speaking of the chalk, says 
that it ‘forms such an assemblage of sedimentary 
deposits as would probably be presented to observa- 
tion if a mass of the bed of the Atlantic, 2,000 feet 
