JURASSIC ZONKS. 19 
are, in the case of the Jurassic rocks, usually distinguished by the 
name of a prominent fossil, and Ammonites where possible are 
selected as indices, because their vertical range is usually more 
restricted than that of other characteristic fossils, different species 
are found in succession, and many of them occur over wide areas. 
Thus we have the zone of Ammonites Bucklandi in the Lower 
Lias ; the zone of A. Parkinsoni in the Inferior Oolite, &c. 
In this definition of a zone it must be borne in mind that no 
one of the species enumerated as belonging to the assemblage, 
maybe confined Avithin the limits assigned to the zone. Even the 
index-species may range, though less commonly above and below 
it, and may be absent locally from the zone it is taken to represent. 
Thus while a zone is a zoological division, and signifies, as 
Professor Tate has remarked,* rather an assemblage of species 
than the range of an Ammonite ; yet the assemblage will be found 
to vary in different areas, as many of the forms that occur will 
have been restricted by the sedimentary conditions. 
The identification of ;i zone must primarily depend upon the 
occurrence of the index-species, and of wide-spread forms that may 
accompany it, or of slightly different but representative species. 
The stratigraphical sequence of the assemblages found in different 
areas is again of the greatest importance in identifying zones and 
in determining correlation. The rocks too in places are barren of 
organic remains, so that the absence of zonal species and their 
accompaniments affords no necessary proof of unconformity. In 
cases where the index-species is rare or unknown, other species 
have locally been taken to mark zones, but on general grounds this 
course is not to be recommended. 
It is important to distinguish between fossil-beds that contain 
a marked abundance of one or more species, and zones ; for zones 
may include one or more fossil-beds, such as those to which atten- 
tion has previously been drawn. 
As noted in the account of the Lias, observations made on a 
large exposed surface of a formation show the variable character 
of the fossils preserved, whereas in a cliff or cutting we can seldom 
explore any great superficial extent of strata ; our observations are 
usually confined within a few inches, or at most a lew feet. Thus 
one observer may find a species only within certain limits, whereas 
another may obtain evidence to fix the range quite differently. 
Many fossils are thus preserved at different horizons, and while in 
one place species may have a restricted vertical range, elsewhere 
the range may be much greater. In most cases the limits 
assigned to species are based on negative evidence, and sub- 
divisions based on the ascertained local range of any particular 
species may have but little value. Zones therefore are not 
always to be identified from the occurrence of any one specimen, 
* Quart. Journ. Geol. Soc., vol. xxiii. p. 300. See also Duncan, Supp. to Brit. 
Foss. Corals, Part iv., No. 1 , pp. 2 to 4 ; Judd. Geol. Rutland, pp. 48, &c., Quart. 
Journ. Geol. Soc., vol. xxxiv. p. 704 ; and H. B. W., Proc. Geol. Assoc., vol. xii. 
p. 295. 
B 2 
