﻿Vol. 6 1.] SEQUENCE IN THE BRISTOL AREA. 185 



If, however, it is found that the zonal indices succeed each other 

 in the same order in two distinct localities, and it is also found that 

 a number of other fossils are common to the two localities, it 

 cannot be assumed as a necessary deduction, and is not a demon- 

 strated fact, that the fossils associated with any particular index in 

 the one place, will be again associated with that index in the 

 second place, or will be distributed in the same way throughout 

 its zone. Even in a small area, there is, in the case of two widely- 

 different classes of organisms, such as Corals and Brachiopods, a 

 small relative displacement of the one group upon the other, at 

 different points of the area. 



For example, if, in the Burrington, Avon, and Tytherington 

 sections, we register the first occurrence of Oaninia, in terms of 

 the brachiopod-sequence at each locality, the event is found to 

 occur at Burrington in the resupinata-siibzone, in the Avon section 

 just above that subzone, and at Tytherington in the laminosa-suh- 

 zone : thus demonstrating a retardation of the coral-sequence upon 

 the brachiopod-sequence, as we proceed from south to north. 



If the amount of relative displacement of one group upon another 

 were accurately charted at different points of any area, it would be 

 possible to estimate the relative acceleration of the one group upon 

 the other, both in direction and amount, and, hence, to deduce the 

 probable association of fossils in a distant locality. 



At an early stage it was found necessary, for purposes of accurate 

 registration and co-ordination, to adopt a series of zonal indices. 

 Those genera, consequently, are selected as zonal indices, the 

 abundance of which throughout definite portions of the series has 

 forced itself upon my attention, when working in the field. 



It was ascertained that there was usually a certain interval, of 

 greater or less extent, in which the indices of two successive zones 

 were found associated together. The faunal overlaps of zones form 

 very definite horizons, and I have consequently designated them 

 Horizon a, /3, etc. 



A series of subzonal indices was next adopted, to designate 

 the parts of a zone, whenever a marked change of fauna naturally 

 called for the subdivision of a zone. For this purpose species- 

 groups were employed, and the particular groups selected have 

 been chosen for one of two reasons : — 



Either (1) they have their maximum development within the portion which 

 they were selected to indicate ; 

 or (2) they are survivors of an earlier fauna, occurring commonly among 

 the members of a later facies. 



It is important, however, to emphasize the fact that the zonal 

 and subzonal indices are merely employed to connote 

 special faunal aggregates, so that the presence of a zone can 

 be definitely asserted, even though the index of that zone has not 

 been discovered. 



