8 INTRODUCTION 



(iv) Oxide of iron, in the form of limonite in some 

 fossils from the Dogger (Inferior Oolite) of York- 

 shire and the Lower Greensand of Potton, etc., 

 and as haematite in fossils from the Carboniferous 

 Limestone of Cumberland. 



(v) In rare cases there are other replacing minerals, 

 such as sulphate of lime, barytes, blende, galena, 

 malachite, vivianite, and spathic iron. 



6. Imprints. The footprints of animals and the 

 impressions of jelly-fishes are sometimes found in the 

 rocks, and these, although forming no part of the animal 

 itself, are nevertheless regarded as fossils. 



In endeavouring to discover the changes which have 

 taken place on the earth in past geological times, the 

 evidence furnished by fossils is of primary importance. 

 Each great group of the stratified rocks, known as a 

 system, is characterised by a particular assemblage of 

 genera and species, some of which are confined to it and 

 enable us to identify the system. In a similar manner, 

 the smaller divisions — the series and stages, are each 

 characterised by the presence of certain fossils, which do 

 not occur above or below. Further, it is found that the 

 fauna of the smallest division (stage or group of beds) 

 is not of uniform character throughout ; although there 

 may be no change in the nature of the rock, some of the 

 species and varieties which are abundant at one level will 

 become rare or will disappear entirely in passing to higher 

 or lower horizons. Consequently, a set of beds may be 

 divided into belts or zones, the general aspect of the fauna 

 of each zone being somewhat different from that of the 

 others, but between these divisions there will be no break 



