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cu ees “DYNAMICAL GEOLOGY. —_—_ [Book IIT. 
much higher rate of erosion than can, as the average, be claimed for 
them.! Then_a slice of about a mile in breadth will require about 
52,800 years for its demolition, ten miles will be eaten away in 
528,000 years, one hundred miles in 5,280,000 years. Now we have 
already seen that, on a moderate computation, the land loses about 
a foot from its general surface in 6000 years, and that by the 
continuance of this rate of subaerial denudation, the continent of 
Europe might be worn away in about 4,000,000 years. Hence, be- 
fore the sea, advancing at the rate of ten feet in a century, could 
pare off more than a mere marginal strip of land, between 70 aud 80 
miles in breadth, the whole land might be washed into the ocean by 
atmospheric denudation. ) 
Some such results as these would necessarily be produced if no 
disturbance took place in the relative levels of sea and land. But in 
estimating the amount of influence to be attributed to each of the 
denuding agents in past times, we require to take into account the 
complicated effects which would arise from the upheaval or depression 
of the earth’s crust. If frequent risings of the land or elevations of 
the sea-floor into land had not taken place in the geological past, 
there could have been no great thickness of stratified rocks formed, 
for the first continents must soon have been washed away. But the 
great depth of the stratified part of the earth’s crust and the abundant 
breaks and unconformabilities among these sedimentary masses, show 
how constantly on the one hand the waste of the land was compensated 
by the result of elevatory movements, while on the other, the con- 
tinued upward growth of vast masses of sedimentary deposits was 
rendered possible by prolonged depression of the sea-bed. 
When a mass of land is raised to a higher level above the sea, a 
larger surface is exposed to denudation. Asa rule a greater rain- 
fall is the result, and consequently also'a more active waste of the 
surface by eubaerial agents. It is true that a greater extent of coast 
line is exposed to the action of the waves, but a little reflection will 
show that this increase will not, on the whole, bring with it a pro- 
pee increase in the amount of marine denudation. For as the 
and rises the cliffs are removed from the reach of the breakers, and 
a more sloping beach is produced on which the sea cannot act with 
the same potency as when it beats against a cliff-line. Moreover, as 
the sea-floor approaches nearer to the surface of the water it is the 
former detritus washed-off the land and deposited under the sea, 
which first comes within the reach of the currents and waves. This 
serves, in some measure, as a protection to the solid rock below, and 
must be cut away by the ocean before that rock ean be exposed anew. 
While, therefore, elevatory movements tend on the whole to accelerate 
the action of subaerial denudation, they in some degree check the 
' It may be objected that this rate is far below that of parts of the east coast of 
England, where the land sometimes loses three or four yards in one year. But on the 
other hand, along the rocky western coast, the loss is perhaps not so much as one foot 
in a century. 
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