284 
IMPEKFECTION OF THE 
Chap. IX. 
observer remember Lyell’s profound remark tbat the 
thickness and extent of sedimentary formations are the 
result and measure of the degradation which the earth’s 
crust has elsewhere suffered. And what an amount 
of degradation is implied by the sedimentary deposits 
of many countries! Professor Eamsay has given me 
the maximum thickness, in most cases from actual 
measurement, in a few cases from estimate, of each 
formation in different parts of Grreat Britain; and this 
is the result:— 
Feet. 
Palasozoic strata (not including igneous beds) .. 57,154 
Secondary strata. .. 13,190 
Tertiary strata . 2,240 
—making altogether 72,584 feet; that is, very nearly 
thirteen and three-quarters British miles. Some of 
the formations, which are represented in England by 
thin beds, are thousands of feet in thickness on the Con¬ 
tinent. Moreover, between each successive formation, 
w’e have, in the opinion of most geologists, enormously 
long blank periods. So that the lofty pile of sedimen¬ 
tary rocks in Britain, gives but an inadequate idea of 
the time which has elapsed during their accumulation; 
yet what time this must have consumed! Good ob¬ 
servers have estimated that sediment is deposited by the 
great Mississippi river at the rate of only 600 feet in a 
hundred thousand years. This estimate has no preten¬ 
sion to strict exactness; yet, considering over what wide 
spaces very fine sediment is transported by the currents 
of the sea, the process of accumulation in any one area 
must be extremely slow. 
But the amount of denudation which the strata have 
in many places suffered, independently of the rate of 
accumulation of the degraded matter, probably offers 
the best evidence of the lapse of time. I remember 
