Boox VI] GENERAL PRINCIPLES. 633 
_ species on each side of a band are peculiar, we naturally infer, if the 
total number of species seems large enough to warrant the inference, | 
that the interval marked by the former break was very much longer 
than that marked by the latter. But we may go further and compare 
by means of fossil evidence the relation between breaks in the succes- 
sion of organic remains and the depth of strata between them. 
Three series of fossiliferous strata, A, C, and H, may occur con- 
formably above each other. By a comparison of the fossil contents 
of all parts of A, it may be ascertained that, while some species are 
-peculiar to its lower, others to its higher portions, yet the majority 
extend throughout the group. If now it is found that of the 
total number of species in the upper portion of A only one-third 
passes up into C, it may be inferred with some probability that the 
time represented by the break between A and C was really longer 
than that required for the accumulation of the whole of the group 
A. It might even be possible to discover elsewhere a thick in- 
termediate group B filling up the gap between A and C. In like 
manner were it to be discovered that, while the whole of the group 
C is characterized by a common suite of fossils, not one of the 
species and only one half of the genera pass up into H, the inference 
could hardly be resisted that the gap between the two groups 
marks the passage of a far longer interval than was needed for the 
deposition of the whole of C. And thus we reach the remarkable 
conclusion that, thick though the stratified formations of a country 
may be, in some cases they may not represent so long a total period 
of time as do the gaps in their succession,—in other words, that non- 
deposition has been in some areas more frequent and prolonged than — 
deposition, or that the intervals of time which have been recorded 
by strata have sometimes not been so long as those which have not 
been so recorded. 
Tn all speculations of this nature, however, it is necessary to 
reason from as wide a basis of observation as possible, seeing that so 
much of the evidence is negative.. Especially needful is it to bear 
in mind that the cessation of one or more species at a certain line 
‘among the rocks of a particular district may mean nothing more 
than that, owing to some change in the conditions of life or of 
deposition, these species were compelled to migrate, or became 
locally extinct at the time marked by that line. They may have 
continued to flourish abundantly in neighbouring districts for a long 
period afterward. Many examples of this obvious truth might be 
cited. Thus ina great succession of mingled marine, brackish-water, 
and terrestrial strata, like that of the Carboniferous Limestone series 
of Scotland, corals, crinoids, and brachiopods abound in the lime- 
stones and accompanying shales, but grow fewer or disappear in the 
sandstones, ironstones, clays, coals, and bituminous shales. An 
observer meeting for the first time with an instance of this dis- 
appearance, and remembering what he had read about “breaks in 
