_ 
614 PALMONTOLOGICAL GEOLOGY. — [Book V. 
may at least tentatively infer that the same kind of climate prevailed 
where that assemblage lived. Such an inference would become more 
aud more unsafe in proportion to the antiquity of the fossils and 
. their divergence from existing forms. 
2. Geological chronology.—Although absolute dates cannot 
be fixed in geological chronology, it is not difficult to determine the 
relative age of different strata. For this purpose the fundamental 
law is based on the “order of superposition” (p. 500). The 
law may thus be defined: in a series of stratified formations the 
older must underlie the younger. It is not needful that we should 
actually see the one lying below the other. If a continuous conform- 
able succession of strata dips steadily in one direction we know that 
the beds at the one end must underlie those at the other, because we 
can trace the whole succession of beds between them. Mare instances 
occur where strata have been so folded by great terrestrial disturbance 
that the younger are made to underlie the older. But this inversion 
can usually be made quite clear from other evidence. The true order | 
of superposition is decisive of the relative ages of stratified rocks. 
The order of sequence having been determined, it is needful to 
find some means of identifying a particular formation elsewhere, 
where its stratigraphical relations may possibly not be visible. At 
first it might be thought that the mere external aspect and mineral 
characters of the rocks ought to be sufficient for this purpose. Un- 
doubtedly these features may suffice within the same limited region 
in which the order of sequence has already been determined. But 
as we recede from that region they become more and more unre- 
liable. ‘That this must be the case will readily appear, if we reflect 
upon the conditions under which sedimentary accumulations have 
been formed. The markedly lenticular nature of these deposits 
has already been described (p. 491). At the present day the sea- 
bottom presents here a bank of gravel, there a sheet of sand, else- 
where layers of mud, or of shells, or of organic ooze, all of which are 
in course of deposit simultaneously, and will as a rule be found to 
shade off laterally into each other. The same diversity of contem- 
poraneous deposits has obtained from the earliest geological periods, 
Conglomerates, sandstones, shales, and limestones occur on all geo- 
logical horizons, and replace each other even on the same platform, 
The Coal-measures of Pennsylvania are represented west of the Rocky 
Mountains by thousands of feet of massive marine limestones. The 
white chalk of England lies on the same geological horizon with 
marls and clays in North Germany, thick sandstones in Saxony, hard 
limestone in the south of France. Mere mineral characters are 
thus quite unreliable save within comparatively restricted areas. 
The solution of this problem was found and was worked out for 
the Secondary rocks of England by William Smith at the end of last 
century. It is supplied by organic remains, and depends upon the 
law that the order of succession of plants and animals has been 
similar all over the world. According to the order of superposition 
