i8 PALEONTOLOGY 



the Bradford clay ; below this the thick mass of oolitic 

 limestone for which Bath is famous, and so on. In 

 Fig. 5 are shown the species of brachiopods that are 

 characteristic of two of these formations. The value of 

 such fossils for arranging the stratified rocks of the earth 

 in systematic order will be realized when we remember 

 that though in a hilly district with many quarries and 

 other openings into the rocks like that around Bath, or 

 along a cliffy coast, it may be quite easy to prove clearly 

 how one group of strata overlies another, yet when we 

 try to follow the strata from place to place and especially 

 into regions where exposures of the rocks are few and 

 poor, there are many difficulties to be met. A stratum 

 may change its character, for example the Forest Marble 

 changes from limestone to clay in Northamptonshire ; 

 or it may "thin out" and disappear altogether; or a 

 "fault" may suddenly shift its position greatly. Amid 

 these difficulties, the finding of distinctive fossils will 

 often save the geological surveyor from a mistake which 

 might perhaps lead someone to sink a mine or bore for 

 water in a wrong place. 



The whole group of animal species found together in 

 the same beds are spoken of as the fauna of those beds, 

 just as the collection of species now living in any given 

 area is the fauna of that area. In past times, as at 

 present, a fauna had a definite habitat or distribution in 

 space ; so that the fact of a fossil fauna in Australia being 

 different from one in England does not prove that they 

 were not contemporaneous. This fact may at first sight 

 seem to make it impossible to apply William Smith's 



