DISTRIBrTION 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 

 heing 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 Eamsay* 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 lie 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. Each 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 life is 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 ; 

 and so it often happens that strata a few yards in thickness, or, 



recently applied by D'Orbigny, Sedgwick, Murchison, and Barrande ; and sonae 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 tj^ie of which was taken from 

 Touraine. 



The term Icenian is proposed for the PUocene strata because their order of succes- 

 sion was first determined by Mr. Charlesworth, in the eastern counties ot England, the 

 coimtry of the Iceni. 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. 



* Anniversarj' Addresses, Q. J. Geol. Soc, vols. xix. and xx. 1863 and 1864, 



