
_ Boox V.] USES OF FOSSILS. 613 
-North America, whence so rich a terrestrial and lacustrine flora and 
fauna have been obtained. 
- (c.) Old sea-bottoms are vividly brought before us by beds of 
marine shells and other organisms. Layers of water-worn gravel and 
sand, with rolled shells of littoral and infra-littoral species, unmis- 
takeably mark the position of a former shore line. Deeper water is 
indicated by finer muddy sediment, with relics of the fauna that 
prevails beneath the reach of waves and ground-swell. Limestones 
full of corals, or made up of crinoids, point to the slow, continuous 
erowth and decay of generation after generation of organisms in 
clear sea-water. 
(d.) Variations in the nature of the water or of the sea- 
bottom may sometimes be shown by changes in the size or shape of 
the organic remains. If, for example, the fossils in the central and 
lower parts of a limestone are large and well-formed, but in the 
upper layers become dwarfed and distorted, we may reasonably infer 
that the conditions for their continued existence at the locality 
_ must have been gradually impaired. ‘The final complete cessation of 
these favourable conditions is shown by the replacement of limestone 
by shale, indicative of the water having become muddy, and by the 
disappearance of the organisms, which had shown their sensitiveness 
to the change. 
(e.) The proximity of land at the time when a fossiliferous 
stratum was in the course of accumulation is sufficiently proved by 
mere lithological characters, as has been already explained; but the 
conclusion may be further strengthened by the occurrence of leaves, 
stems, and other fragments of terrestrial vegetation which, if found 
in some numbers among marine organisms, would make it improbable 
that they had been drifted far from land (see, however, p. 439). 
_ (f.) The existence of different conditions of climate in former 
geological periods is satisfactorily demonstrated from the testimony 
of fossils. ‘Tbus an assemblage of the remains of palms, gourds, and 
melons, with bones of crocodiles, turtles, and sea-snakes, proves a 
sub-tropical climate to have prevailed over the south of England 
in the time of the older Tertiary formations. On the other hand, 
the extension of an intensely cold or arctic climate far south into 
Europe during post-Tertiary time can be shown from the existence 
of the remains of arctic animals even in the south of England and of 
France. This isa use of fossils, however, where great caution must be 
used. We cannot affirm that, because a certain species of a genus 
lives now in a warm part of the globe, every species of that genus 
must always have lived in similar circumstances. The well-known 
example of the mammoth and woolly rhinoceros having lived in the 
cold north, while their modern representatives inhabit some of the 
warmest regions of the globe, may be usefully remembered as a 
warning against any such conclusions. When, however, not one 
fossil merely, but the whole assemblage of fossils in a formation finds 
its modern analogy in a certain general condition of climate, we 
