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. 

 Having 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, phyllite?, 

 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 have 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 Archaean 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 talc-schists the middle, 

 and phyllites (primitive schists) the youngest division of this group. The so- 



