346 
president’s address. 
“zones”; these are stages in the history of life on the earth that are 
characterised by one or more species whose geographical distribution 
was wide, but whose duration in time was comparatively restricted. 
Thus among the older rocks we find certain strata characterised 
by particular forms of Graptolites, and similar species or genera 
succeed one another in the same order over areas now widely 
separated. In the Lias again we find a sequence in the species of 
Ammonites that corresponds over large tracts. It is concluded 
that the strata yielding these successively similar forms of life are 
practically contemporaneous ; but from what has been said of the 
possible evolution of identical species in different areas, it is 
necessary to consider the general characters of the fauna that 
constitutes these zones. 
In the Chalk different assemblages of fossils characterise 
successive portions of the formation. Some species, indeed, may 
range through the entire formation, but the particular assemblages, 
named for convenience after some prominent species, are found 
generally to correspond over large areas.'" 
It must not be supposed that these zones are anything more than 
pages in the history of the earth ; but they afford the only means 
wo have of comparing in age, formations that are remote one from 
another. They are not in themselves to he separated by definite 
planes of division, any more than English history is interrupted 
by a change of sovereign or by a change of government. .New 
forms of life come to the front and oust others ; and such changes, 
as in human history, while more marked over limited areas, yet 
blend in the general sequence of events. Ordinary events go on 
much the same. 
Each country must have its separate grouping of strata, and 
although we employ all the world over such general terms as 
Silurian, Jurassic, and Cretaceous, yet so far as the strata are 
concerned we can only look upon the correlation as broad and 
general — probably exact within certain limits ; but we cannot 
define those limits as they shade away irregularly, and must vary 
in different areas. We see this in our own country, for at 
* Sec Proo. Geol. Assoc, vol. xii. p. 295. 
