12 LIAS OF ENGLAND AND WALES: 
certain kinds of rock are characterized by forms of life suited 
to the sedimentary conditions they represent, yet the range of 
some species and the incoming of new ones may be quite inde- 
pendent of sedimentary changes. In short, the palseontological 
changes frequently do not coincide with the lithological changes. 
The difficulties in classification and correlation therefore appear 
great, but they are only so because it is sought to make divisions 
where none exist in Nature. Neither by lithological characters 
nor by fossils can we fix persistent planes of demarcation in a 
conformable series of strata, but both are of service in helping 
to fix and correlate those artificial divisions which it is necessary 
to adopt for the purpose of tabulating our knowledge. 
It is important to determine the physical structure of each 
district; to represent on maps the superficial area occupied by 
each group of rocks, and to show their relation to the form of the 
ground. Hence the subdivisions of each geological system must 
be mainly stratigraphical ; such divisions are permanent in the 
area to which they refer, although distinct divisions may be 
necessary in the same system in other areas. Purely palseonto- 
logical divisions must always be more or less vague and indefinite 
so far as their limitations are concerned, but the artificial division 
of the strata into zones irrespective of the main rock-divisions 
is of essential service in correlating the strata of different areas. 
To these zones further reference will be made. 
The position of a barren group of rocks can be usually ascer- 
tained by working out its relations to beds above and below, in 
which beds some distinctive fossils may be obtained ; or the strata 
themselves may in themselves present a sequence of rocks of 
distinctive character, such as would determine their correlation 
with other formations, exhibiting a similar repetition. The sands 
of the Middle Lias and the Midford Sands cannot always 
be distinguished, nor the Lower and Upper Lias clays, nor 
certain beds in the Inferior Oolite an:l Great Oolite. Many 
other illustrations might be given ; but when we work out the 
stratigraphical relations and find a sequence of Marls-tone, Upper 
Lias clay, and Midford Sands; or of Fullers Earth, Great 
Oolite and Forest Marble, the position of the doubtful strata can 
be determined. 
Sections of the Strata. 
In many instances the published records of early date furnish 
our only information of the strata of certain localities. In future 
times this is likely to be more arid more the case, for the number 
of open sections every year decreases. This is a fact although a 
melancholy one for geologists. Railways have indirectly been the 
cause. It arises partly on account of the introduction of hard 
road-metal to districts and villages which in former years depended 
entirely on such local stone as was to be found. The " Mendip 
granite " (as the Carboniferous Limestone is commercially mis- 
called), the Hartshill stone, the Charnwood Forest and Mount 
Sorrel rocks, and the; Clec Hill Dhu stone, are responsible for the 
