54 PRINCIPLES OF PALZAONTOLOGY. 
Sedimentary Rocks. Plants, again, even when undoubtedly 
such as must have grown on land, do not prove that the bed 
in which they occur was formed on land. Many of the remains 
of plants known to us are extraneous to the bed in which they 
are now found, having reached their present site by falling into 
lakes or rivers, or being carried out to sea by floods or gales of 
wind. There are, however, many cases in which plants have 
undoubtedly grown on the very spot where we now find them. 
Thus it is now generally admitted that the great coal-fields 
of the Carboniferous age are the result of the growth zz sztu 
of the plants which compose coal, and that these grew 
on vast marshy or partially 
submerged tracts of level 
alluvial land. We _ have, 
however, distinct evidence 
of old land-surfaces, both in 
the Coal-measures and in 
other cases (as, for instance, 
in the well-known “ dirt- 
bed” of the Purbeck series). 
When, for example, we find 
the erect stumps of trees 
standing at right angles 
to the surrounding strata, 
we know that the surface 
through which these send 
their roots was at one time 
the surface of the dry land, 
or, in other words, was an 
ancient soil (fig. 19). 
In many cases fossils en- 
able us to come to important 
= conclusions as to the climate 
Fig. 19.—Erect Tree containing Reptilian of the period in which they 
SU aed seine Nova Scotia. (After lived, but only a few in- 
stances of this can be here 
adduced. As fossils in the majority of instances are the re- 
mains of marine animals, it is mostly the temperature of the 
sea which can alone be determined in this way ; and it is import- 
ant to remember that, owing to the existence of heated currents, 
the marine climate of a given area does not necessarily imply a 
correspondingly warm climate in the neighbouring land. Land- 
climates can only be determined by the remains of Jand-ani- 
mals or land-plants, and these are comparatively rare as fossils. 
It is also important to remember that all conclusions on this 
