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 yast accumulations of the 
whole Silurian series.’ The lapse of time isin most cases further 
marked by extensive denudations of strata. During the Palzo- 
zoic age ten physical breaks are known, six of which occu 
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 corsider- 
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 ina 
sandstone; and even in comparing similar strata we must con- 
sider the probability of their haying 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 haye died out, one by 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. Hach 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 be obtained by considering that the deposits in the valley of the Mississippi, 
estimated to represent 100,000 years, have been accumulated since the era of many 
existing shells. The same may be said of the elevation cf Mont Blanc, the formation 
of the Mediterranean Sea, and other grand physical vents. The great cities of anti- 
quitv—Rome, Corinth, and Egyptian Thebes—stand upon raised sea beds, or alluvial 
deposits, containing recent shells. 
