46 CLASSIFICATION OF TERTIARY FORMATIONS 
lected and compared with those of other countries, and we are 
almost entirely ignorant of many deposits known to exist in 
Spain and Portugal. 
The theoretical views developed in the last chapter, respect- 
ing breaks in the sequence of geological monuments, will ex- 
plain our reasons for anticipating the discovery of intermediate 
gradations as often as new regions of great extent are explored. 
Comparative value of different classes of organic remains. 
In the mean time, we must endeavour to make the most syste- 
matic arrangement in our power of those formations which are 
already known, and in attempting to classify these in chrono- 
logical order, we have already stated that we must chiefly 
depend on the evidence afforded by their fossil organic con- 
tents. In the execution of this task, we have first to consider 
what class of remains are most useful, for although every kind 
of fossil animal and plant is interesting, and cannot fail to 
throw light on the former history of the globe at a certain 
period, yet those classes of remains which are of rare and casual 
occurrence, are absolutely of no use for the purposes of general 
classification. If we have nothing but plants in one assemblage 
of strata, and the bones of mammalia in another, we can ob- 
viously draw no conclusion respecting the number of species of 
organic beings common to two epochs ; or if we have a great 
variety, both of vertebrated animals and plants, in one series, 
and only shells in another, we can form no opinion respecting 
the remoteness or proximity of the two eras. We might, per- 
haps, draw some conclusions as to relative antiquity, if we could 
compare each of these monuments to a third ; as, for example, if 
the species of shells should be almost all identical with those 
now living, while the plants and vertebrated animals were all ex- 
tinct ; for we might then infer that the shelly deposit was the 
most recent of the two. But in this case it will be seen that 
the information flows from a direct comparison of the species of 
corresponding orders of the animal and vegetable kingdoms, — 
of plants with plants, and shells with shells ; the only mode of 
