4 ELEMENTS OF PALAEONTOLOGY 
detritus on dry land by the agency of winds. The distribution of fossils 
among stratified rocks is by no means promiscuous, and neither do all rocks 
contain the same species ; but, on the contrary, each separate stratigraphical- 
complex, and frequently even single beds and layers, are characterised by 
certain particular suites of fossils. The older the rock, the more strikingly 
different from recent organisms are its fossil remains ; the younger the forma- 
tion, the greater is their resemblance. Now, since experience shows that 
contemporaneous deposits which have been laid down under similar conditions 
(as, for example, in salt or in fresh water) contain identical or at least very 
similar fossils, the latter furnish us with an infallible guide, taken together 
with the local stratigraphic succession, for determining the relative age of a 
given formation, Furthermore, a knowledge of the fossils occurring in homo- 
taxial deposits enables us to reconstruct the various palaeofaunas and palaeo- 
floras which have existed on our planet at different periods in its history. 
Haying determined the chronological succession of the clastic rocks by means 
of their superimposition and their characteristic or index-fossils, they may be 
divided up into still smaller series, each one of which is characterised by a 
particular assemblage of organic remains. In the main, then, palaeontology is 
the ultimate foundation of historical geology. 
Excluding the oldest metamorphic rocks (gneiss, mica schists, phyllites, 
etc.) which are destitute of fossils, and concerning whose origin there is still 
great difference of opinion, the total thickness of the sedimentary rocks 
amounts to 20,000—30,000 metres. The building up of this prodigious 
pile of rock must haye extended over an inconceivably long time, whose dura- 
tion cannot even approximately be estimated, since we are without data as 
to the rate of deposition in former periods, and since the beginning, culmina- 
tion, and end of geological epochs cannot be correlated with astronomical 
events. 
Since, however, the earth has been inhabited in former times by very 
different creatures from those now living; since successive palaeofaunas and 
palaeofloras follow one another everywhere in the same order; and since, 
furthermore, in certain formations the greater part or even the total number 
of species appear and disappear in a body, so that one fauna or flora is 
replaced almost in its entirety by the next following; it is obvious that the 
sedimentary rocks may be subdivided into a number of longer and shorter 
time measures, which may be designated by particular names. The beginning 
and end of such periods (group, system, or formation, series or section, stage, 
zone, or bed) is usually indicated by local interruptions in the deposition, 
occasioned by variations in sea-level, volcanic eruptions, or by other causes ; 
and such disturbances are usually accompanied by changes in the flora and 
fauna. The now generally accepted subdivision of the secondary rocks is 
represented in the table on page 5, in which it should be noted that only the 
first three columns are of universal significance, while the last two apply only 
to European conditions. 
The rocks of the Archacan Group amount to 40,000—60,000 metres in 
thickness. They belong to the oldest and longest period in the history of 
our planet, and are remarkable for their schistose and crystalline structure, as 
well as for the total absence of fossils. In order of stratigraphy, gneiss com- 
prises for the most part the oldest, mica, chlorite, and tale-schists the middle, 
and phyllites (primitive schists) the youngest division of this group. The so- 
