120 MANUAL OF THE MOLLUSCA. 



more notably still, the absence of these strata, may serve to 

 indicate a period of time as great as the vast accumulations of the 

 ■whole Silurian series." The lapse of time is in most cases further 

 marked by extensive denudations of strata. During the Palaeo- 

 zoic age ten physical breaks are known, six of which occur 

 before we reach the Devonian formation. In every case but 

 one (and in that the rocks are almost entirely devoid of animal 

 remains), there is an entire change in the species and a consider- 

 able change in the genera. The breaks in the Secondary period 

 are less marked and less numerous, amounting to about four ; 

 and they are still less marked in the Tertiary period. 



We have seen that distinct faunas may be separated by narrow 

 barriers in existing seas ; and differences almost as great may 

 occur on the same coast-line without the interposition of any 

 barrier, merely in passing from a sea-bed of rock and weed to 

 one of sand or mud, or to a zone of different depth. It would 

 be unreasonable to expect the same fossils in a limestone as in a 

 sandstone ; and even in comparing similar strata we must con- 

 sider the probability of their having been formed at different 

 depths, or in distinct zoological provinces. 



The most careful observations hitherto made, under the most 

 favourable circumstances, tend to show that all sudden altera- 

 tions have been local, and that the law of change over the whole 

 globe and through all time has been gradual and uniform. 

 The hypothesis of Sir C. Lyell, that species have been created, 

 and have died out, one hy one, agrees far better with facts, than 

 the doctrine of periodic and general extinctions and creations. 



As regards the zoological value of the " formations," we shall 

 be within the truth if we assume that those already established 

 correspond in importance with geographical provinces ; for at 

 least half the species are peculiar, the remainder being common 

 to the previous or succeeding strata. This will give to each 

 Geological period a length equal to three times the average 

 duration of the species of marine shells.* 



The Distribution of the Species in the Strata (or in Time) is like 

 their distribution in space. Each is most abundant in one 

 horizon, and becomes gradually less frequent in the beds above 



* The exact value of these periods cannot be ascertained, but some notion of their 

 length may ho obtained by considering that the deposits in the valley of the Mississippi 

 estimated to represent 100,000 years, liave been accumulated since the era of many 

 existing shells. The same may be said of the elevation of Mont Blanc, the formatioa 

 of the Mediterranean Sea, and otlier grand physical . vents. The great cities of anti- 

 quity—Rome, Corinth, and Egyptian Thebes— stand upon raised sea-beds, or alluvial 

 dep sts, containing recent shells. 



