482 Eev. J. F. Blake- — Aggregate Deposits and Zones. 



the specific cliaracter of the fossils. There are, moreover, cases in 

 which, one method or the other is the only one available, and there- 

 fore a widely applicable chronology must be founded upon both. It 

 is the object of this paper to clear away a difficulty that sometimes 

 arises in harmonizing the two methods. 



Every species of fossil, considered as an inhabitant of the various 

 parts of the earth to which it at any time extends, has a definite 

 period of existence ; and the same is true of every group of allied 

 species, such as used to go in former times under one specific name ; 

 but in this case the range in space and time is greater. It follows 

 from this that the epoch of every fossiliferous deposit must not only 

 lie within the period of every fossil it contains, but also within that 

 part where all the periods overlap. The deposit is like a document 

 containing the signatures of numerous individuals, born at different 

 dates and having different lengths of life ; it must have been drawn 

 up during the time that they were all alive together. 



It was on this principle, which regards the whole of a fauna of 

 a bed, that the classification of strata by zones was fii'st established 

 by Oppel. Unfortunately this principle of assemblages does not lend 

 itself well to nomenclature, and for the purpose of naming the zone 

 some particular fossil had to be selected. Such a fossil should be an 

 abundant one, restricted as to range in time, but little restricted as to 

 range in space. The selection, however, is arbitrary and may be 

 unsuitable, but to the true student of zones it is a matter of com- 

 parative indifference ; the zonal assemblage of fossils can be 

 recognized without the presence of the particular name-giving fossil. 



Amongst the Jurassic rocks, for the study of which the method of 

 zones was first introduced, the greater number of name-giving fossils 

 have been selected from the ammonites, which to a large extent best 

 fulfil the necessary conditions ; and this has apparently led in more 

 recent years to an almost exclusive study of these types for the 

 purposes of correlation, and to the practical assumption that the zone 

 and the zone-naming fossil are strictly coterminous. Theoretically 

 the life-history of a species is divided into three parts — its rise, 

 culmination, and decline ; or, as they are technically called, the 

 epacme, acme, and paracme — and the zone is supposed to be the 

 deposit formed during the acme. Practically, however, it is seldom 

 possible to trace these three periods, and the presence or absence of 

 the ammonite is all that can be stated. Thus the original idea of 

 the zone is entirely altered. It is characterized no longer by an 

 assemblage of fossils of all classes, each having its own range in 

 time and each contributing to our estimate of age, but by the range 

 of a single ammonite. 



So long as the original idea of a zone was held, the occurrence 

 of two of the name-giving fossils in the same bed caused no 

 trouble beyond the suspicion that perhaps the fossil to be used for 

 the name might have been better selected, but under the new idea 

 such a combination is contrary to the practical assumption on which 

 it rests. It becomes, therefore, incumbent on those who make this 

 assumption to show that the combination of two or more zonal 



