DISTRIBUTION OF MOLLUSCA IN TIME. 119 
According to MM. Agassiz and D’Orbigny, all, or nearly all 
the fossils of each formation are peculiar; very few species 
being supposed to have survived from one period to another. 
Sudden and entire changes of this kind only take place when 
the nature of the deposit is completely altered—as when sands 
or clays rest upon chalk—and in these instances there is usually 
evidence (in the form of beds of shingle, or a change of dip) that 
an interval must have elapsed between the completion of the 
lower stratum and the commencement of the upper. 
Professor Ramsay* has discussed this subject at considerable 
length. He endeavours to prove that where we have a com- 
plete succession of rocks the species die out and appear 
gradually and almost imperceptiby; that where there is any 
sudden change in the fauna, it is always accompanied by an 
unconformity in the rocks—that is, the rocks do not he evenly 
on one another, but the lower one shows an eroded surface, or 
its stratifications are not parallel with those of the upper rock. 
A break in the current of animal life is believed to be always 
accompanied by a break in the succession of rocks. Hach break 
marks a lapse of time during which no deposition of mud, &c., 
took place on the area marked by the break. As it is assumed 
that the change of specific forms has proceeded at a uniform 
rate throughout geological time, it is argued that the greater the 
difference in the fauna, the longer was the time indicated by the 
break. ‘‘I cannot resist the general inference that in cases of 
superposition, in proportion as the species are more or less con- 
tinuous—that is to say, as the break of lifeis partial or complete, 
first in the species, but more importantly in the loss of old and 
the appearance of new allied or unallied genera—so was the 
interval of time shorter or longer that elapsed between the close 
of the lower and the commencement of the upper formation ; 
»nd so it often happens that strata a few yards in thickness, or, 
recently applied by D’Orbigny, Sedgwick, Murchison, and Barrande ; and some are 
adopted from popular usage. Geographical names, and those derived from charac- 
_ teristic fossils have been found the best, but no complete scheme of zoological nomen- 
clature has been framed. 
The epithet “Turonien’’ (25) is rejected, because it conveys the same meaning 
with “ Falunian” (29), or Middle Tertiary, the type of which was taken from 
Touraine. 
The term /cenian is proposed for the Pliocene strata because their order of succes- 
sion was first determined by Mr. Charlesworth, in the castern counties ot England, the 
country of the IcrENI. We have left the table as it stood in the first edition of this 
work; but we should mention here that one formation should be placed at the head, 
viz., the Laurentian, and the beds deposited during and since the glacial epoch at the 
foot. 
* Anniversary Addresses, Q. J. Geol. Suc., vols, xix, and xx, 1863 and 1864, 
fa 
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