CORRELATION OF STRATA. J 1 
persistent over large areas, and its subdivisions may be very 
inconstant. Such is the case with the Inferior Oolite and the 
Corallian rocks. Sections recorded in different parts of the same 
quarry vary, as do the records taken at different times. 
The Jurassic rocks thus exhibit certain phases of the sea- 
bottom or of sedimentation, that endured for longer or shorter 
periods, the conditions changing more or less irregularly and at, 
intervals over the area we have under consideration. The sub- 
divisions of the rocks indicate the domination of certain physical 
conditions over a particular area, and we have to consider the 
probability of certain sedimentary conditions (or formations) 
enduring much longer in one area than in another. We may, for 
instance, have passage-beds on somewhat different horizons, for a 
formation, such as the Corallian, may merge upwards nnd down- 
wards into the rocks above find below, gradually, and yet whwi 
traced for some distance, so irregularly that different planes of 
division (in point of actual age) may be taken in different 
localities. This is evidently the case with certain divisions of the 
Jurassic rocks, so that precise correlation is not possible.* To 
some extent palaeontology comes to our aid, although for several 
reasons we cannot depend upon it for fixing the limits of strati- 
graphical subdivisions. 
Each main division of the rocks has been found to yield an 
assemblage of organic remains more or less characteristic, some 
species being (so far as is known) peculiar, others especially 
abundant. Hence when the stratigraphical succession of the 
main rock-divisions has been established in one area, and their 
fossils have been determined, we have the means of identifying 
distant or isolated masses of rock by their organic remains, a 
fact of especial importance in reference to strata penetrated 
in deep borings, or thrown out of their normal position by 
faults or other disturbances. 
The entire series of rocks being locally so intimately linked 
together, it is natural that the series of organic remains should 
also be connected ; and this is really the case, for most of the 
genera and some of the species range through many formations, 
each species indeed, so far as we know, having a varied geological 
and geographical range. These organic remains are for con- 
venience grouped into "zones," or assemblages characterized by a 
genus or species of wide-spread occurrence ; but the limits of 
these zones are not to be rigidly defined, for as Professor Judd 
has remarked, " the transition from one fauna to another appears 
to have been in almost every instance a gradual one, the several 
species disappearing individually, and not in groups. "f 
It must be remembered that palseontologicul divisions are useful 
only when fossils are to be obtained, whereas in a large number 
of exposures the rocks yield few or no fossils, or perhaps none 
indicating a special horizon. Moreover, while we find that 
* See also remarks by Prof. Judd, Geol. Rutland, pp. 50, &c. 
f Geol. Rutland, p. 57. 
