THE CAMBRIAN PERIOD. 243 



succession of faunas in different regions is believed to be in a general 

 way the same, likeness of fossils in the formations of different regions is 

 thought to be evidence of their approximate contemporaneity. Fossils, 

 therefore, are but a means to an end, so far as the correlation of strata 

 is concerned. It is not to be inferred, however, that the Olenellus 

 fauna ceased to exist at exactly the same time in different regions of 

 the same continent, much less in different continents, and the variations 

 from a true chronological classification which the correlation by fossils 

 involves cannot be avoided. . _ f _J 



Upper limit of- the Cambrian system.— Not only have the three 

 principal faunal stages, of jthe t)ambrian~been agreed upon, but it has 

 been further agreed to draw the division line between the Cambrian 

 system and that which overlies it at the horizon where the Olenus fauna 

 en|s. The upper^pit-of the-Olenus fauna is therefore the. upper limit 

 of the CambriarT system, and correspondsUn time with the end of the 

 Cambrian period. 



At first thought this division may seem arbitrary, but in reality 

 it is not without a natural basis. The Upper Cambrian formations 

 are occasionally unconformable beneath the next succeeding ones, 

 and in such places the division is a natural one. Much more commonly, 

 the next system is conformable with the Upper Cambrian] but even in 

 conformable series, the faunal changes are more rapid in some parts 

 than in others, and the horizon where the Olenus fauna ends is one 

 where these changes are relatively abrupt. The horizons of rapid 

 change in the character of the fossils may represent times _of geographic 

 changes, from which changes in sedimentation and life resulted. 

 Throughout geological history great faunal changes have followed 

 changes in the relations of land and sea. 



Fig. 97 will help to make clear the principles here set forth. The 

 vertical space of the diagram may be taken to represent the Cambrian 

 system, and a part of the system next above. The fauna found at the 

 bottom, a, undergoes gradual change in the overlying beds up to the 

 horizon b. In the beds next following, b to c, the faunal changes are 

 more rapid, and these beds represent the transition from the Lower 

 to the Middle Cambrian. From c to d the Middle Cambrian fauna is 

 found, though undergoing gradual change with successive beds. Be- 

 tween d and e the changes in the fossils are again more marked, and 

 these beds represent the transition from the Middle Cambrian to the 



