the succession of organic remains.’ 
Boor V.] IMPERFECTION OF GEOLOGICAL RECORD. 621 
be observed of any considerable change of physical conditions at the 
time of deposit. A group of quite conformable strata, having the same 
general lithological characters throughout, may be marked by a great 
discrepance between the fossils above and below a certain line. A 
few species may pass from the one into the other, or perhaps every 
species may be different. In cases of this kind, when proved to be 
~ not merely local but persistent over wide areas, we must admit, not- 
withstanding the apparently undisturbed and continuous character of 
the original deposition of the strata, that the abrupt transition from 
the one facies of fossils to the other must represent a-long interval of 
time which has not been recorded by the deposit of strata. Professor 
Ramsay, who called attention to these gaps, termed them “breaks in 
*+ ‘They occur abundantly among 
the Paleozoic and Secondary rocks which by means of them can he 
separated into zones and formations. It is obvious, of course, that 
even though traceable over wide regions, they were not general over 
the whole globe. There have never been any universal interruptions 
in the continuity of the chain of being, so far as geological evidence 
ean show. The breaks or apparent interruptions existed only in the 
sedimentary record, and were produced by geographical changes of 
various kinds, such as cessation of deposit from failure of sediment 
owing to seasonal or other changes; alteration in the nature of the 
sediment or character of the water; variations of climate from what- 
ever cause; more rapid subsidence bringing successive submarine 
- zones into less favourable conditions of temperature, &c. ; and volcanic 
discharges. The physical revolutions, which brought about the 
breaks were no doubt sometimes general over a whole zoological 
province, more frequently over a minor region. Thus at the close of 
the Triassic period the inland basins of central, southern, and western 
Kurope were effaced, and another and different geographical phase 
was introduced which permitted the spread of the peculiar fauna of 
the “ Avicula contorta zone” from the south of Sweden to the plains 
of Lombardy, and from the north of Ireland to the eastern end of the 
Alps. This phase in turn disappeared, to make way for the Lias with 
its numerous “ zones,’ each distinguished by the maximum develop- 
ment of one or more species of ammonite. ‘These successive geo- 
graphical revolutions must in many cases have caused the complete 
extinction of genera and species possessing a small geographical 
range. 
Tiron all these facts it is clear that the geological record, as it now 
exists, is at the best but an imperfect chronicle of geological history. 
In no country is it complete. ‘The lacune of one region may be 
supplied from another; yet in proportion to the geographical 
distance between the localities where the gaps occur and those whence 
the missing intervals are supplied, the element of uncertainty in our 
reading of the record is increased. The most desirable method of 
research is to exhaust the evidence for each area or province, and to 
1 Q. J. Geol, Soc, xix. p. 36. 
