ODLING : CORRELATION OF THE UPPER AND MIDDLE OOLITES. 283 
Where such a break occurs, the higher beds rest either on a pebble bed 
or on an eroded surface of the underlying bed. The purely local 
faunas such as that of the Astartien, Pterocerieii and Hartwell Clay 
are due entirely to local conditions, and are no more than the fossil 
equivalents of the local faunas that may be found to-day on the sea- 
floor. 
When free-swimming animals, such as Ammonites, are taken as 
2onal indices instead of the more sedentary types such as oysters and 
Bchinoderms, it is seen that the general stratigraphical terms such as 
Coral Kag, Calcareous Grit, Kellaways Beds, etc., are of no more than 
local value, and that the occurrence of a similar lithological and 
palseontological group in two districts need not by any means mean 
contemporaneity in deposition, but is due to purely local conditions. 
At the same time, correlation by Ammonites must not be carried 
to an extreme. The absence of a particular Ammonite need not neces- 
sarily mean the non-deposition or contemporaneous erosion of beds 
of this particular age, but merely that this form was not so rapidly 
distributed. The fact that absolute repetition of Ammonites does not 
appear to occur, whereas their more sedentary associates frequently are 
repeated at different horizons, is suggestive and tends to place more 
faith in their value for purposes of correlation. 
When considering the Jurassic Series over large areas the fact of 
there being in reality only two great breaks in the Series — ^the Bathonian 
Transgression and the Callovien Transgression — is forced upon one, 
and the desirability of a regrouping of the series without reference to 
such terms as Portlandian, Kimeridgean, etc., names that can only be 
associated with local areas, becomes obvious. 
Should this be adopted, we should then have the following general 
groups : — 
Upper Jurassic 
jAll below the Wealden down to and including 
I the Cornbrash. 
Middle Jurassic i 
(All below the Cornbrash down to and including 
[ the zone of Parkinsonia parJci)isoni. 
Lower Jurassic 
All below the zone of Parkinsonia parkinsoni 
down to and including the Khaetic. 
