THE CLASSIFICATION OF THE TERTIARY DEPOSITS. 123 
sisted in tracing tlie beds, as far as possible, by tbeir outcrops 
at the surface, and determining tbeir superposition in sections, 
falling back where these methods failed them on 4 the identi- 
fication of the strata by their organic remains/ In the system 
of nomenclature which they adopted, however, the discoverers 
of the Palaeozoic succession introduced a noteworthy improve- 
ment on the method of their great teacher. Instead of adopting 
names based on the accidental characters of the strata, such as 
4 Lias/ 4 Cornbrash/ and 4 Gault/ they employed local names, 
calling each larger or smaller group of beds after the place in 
which it was found most typically developed, as the 4 Aymestry 
Limestone/ the 4 Bala Formation/ and the 4 Devonian System/ 
This method has been found so convenient in practice, that 
similar local names have since been given by geologists to 
many of the divisions of the Secondary strata which had been 
established by William Smith. 
It may, at first sight, appear that in classifying the strata 
above the Chalk, geologists ought to have followed the same 
methods and adopted the same principles of notation as had 
proved so successful in dealing with the older deposits ; and it 
must be admitted that such a course would have produced a 
uniformity and simplicity in our scheme of geological nomen- 
clature which does not now exist. But a little consideration 
will convince us that these principles of classification and 
nomenclature were not abandoned in the case of the Tertiary 
strata without good cause. 
The Tertiary deposits differ from those of Secondary and 
Palaeozoic age in several very important particulars. While the 
older strata are always of marine and estuarine origin, the 
Tertiaries include lacustrine and terrestrial beds, often of great 
thickness. The cause of this difference is found in the fact 
that, while in the case of the Tertiary deposits, the sea-beds 
have, perhaps, only once been elevated into dry land since the 
beds were accumulated, in the case of the older formations they 
have been subjected to many successive subsidences and eleva- 
tions and from the denudation which took place during these 
movements only the deeper -water and more widely-spread 
sediments have been able to escape. It has been well remarked 
by Mr. Darwin that 4 nearly all our ancient formations, which 
are throughout the greater part of their thickness rich in fossils, 
have been formed during subsidence ; ’ and it is well known to 
all geologists that as we go backwards in the geological series 
we soon find that nearly all traces of terrestrial accumulations 
disappear ; we next lose all relics of fluviatile and lacustrine 
deposits ; then we find littoral marine deposits becoming rarer 
and rarer ; till at last, among the most ancient strata, we seldom 
find any but such as must have been laid down in deep water, 
