612 PALMONTOLOGICAL GEOLOGY. — [Book V. _ 
the whole, been in inverse proportion to its perfection. The more 
complex its structure, the more susceptible has it been of change, 
and consequently the less likely to be able to withstand the influences 
of changing climate, and other physical conditions. A living species 
of foraminifer or brachiopod, endowed with comparative indifference 
to its environment, may spread over a vast area of the sea-floor, and the 
same want of sensibility enables it to endure through the changing 
physical conditions of successive geological periods. It may thus 
possess a great range, both in space and time. Bunt a highly- 
specialized mammal is usually confined to but a limited extent of | 
country, and to a narrow chronological range. , 
iv. Uses of Fossils in Geology.—Apart from their profound 
interest as records of the progress of organized being upon the 
earth, fossils serve two main purposes in geological research: (1) to 
throw light upon former conditions of physical geography, such as 
the presence of land, rivers, lakes, and seas, in places where they 
do not now exist, changes of climate, and the former distribution of 
plants and animals; and (2) to furnish a guide in geological © 
chronology whereby rocks may be classified according to relative date, 
and the facts of geological history may be arranged and interpreted 
as a connected record of the earth’s progress. 
1. Changes in Physical Geography.—A few examples 
will suffice to show the manifold assistance which fossils furnish to 
the geologist in the elucidation of ancient geography. 
(a.) Former land-surfaces are revealed by the presence of 
tree-stumps in their positions of growth, with their roots branching 
freely in the underlying stratum, which, representing the ancient — 
soil, often contains leaves, fruits, and other sylvan remains, together 
with traces of the bones of land animals, remains of insects, land- 
shells, &c. Ancient woodland surfaces of this kind, found between | 
tide-marks, and even below low-water line, round different parts of 
the British coast, unequivocally prove a subsidence of the land 
(p. 281). Of more ancient date are the “ dirt-beds” of Portland, 
which, by their layers of soil and tree-stumps, show that woodlands of 
cycads sprang up over an upraised sea-bottom and were buried beneath 
the silt of a river or lake. Still further back in geological history 
come the numerous coal-growths of the Carboniferous period, pointing 
to wide jungles of terrestrial or aquatic plants, like the modern man- 
grove swamps, which were submerged and covered with sand or silt. 
().) he former existence of lakes can be satisfactorily proved 
from beds of marl or lacustrine limestone full of freshwater shells, 
or from fine silt with leaves, fruits, and insect remains. Such 
deposits are forming abundantly at the present day, and they occur 
at various horizons among the geological formations of past times. 
The well-known nagelflue of Switzerland—a mass of conglomerate 
attaining a thickness of fully 6000 feet—can be shown from its 
fossil contents to be essentially a lacustrine formation. Still more 
important are the ancient Eocene and Miocene lake-formations of 
