484 GEOLOGY 



of the species of the three faunas are different, that with a consid- 

 erable number of species to judge from, their separation is possible 

 by those familiar with Cambrian fossils. 



In the discrimination of any of these faunas, an analogy with 

 living animals is suggested. The present faunas of North and 

 South America are reasonably distinct; but it does not follow that 

 there are no species in common. Given a single animal, even an 

 expert might not be able to say that it was from the one continent 

 or the other, though with certain species even this might be done; 

 but if a large number of animals from either continent are avail- 

 able, it is possible to determine to which continent, that is to 

 which geographic fauna, they belonged. So with the several 

 Cambrian faunas. They have some species in common, and such 

 species do not distinguish the groups of strata which contain them 

 from one another. But certain species are found only in the 

 Lower, certain other species only in the Middle, and still others only 

 in the Upper part of the system, and these species serve to dis- 

 tinguish the principal divisions. 



Sequence of faunas based on stratigraphy. It is not to be un- 

 derstood that rocks which contain such faunas are classed together 

 simply because they contain certain fossils. This is not the reason, 

 or at least not the principal reason for grouping them together. 

 The order of sequence of faunas is first determined by the super- 

 position of the strata. The Lower Cambrian fauna could not be 

 known to be older than the Middle Cambrian fauna, if the beds 

 containing the former did not underlie beds containing the latter. 

 In other words, the primary basis for correlation by means of fossils 

 is stratigraphy. 



Sedimentation in the Cambrian Period 



Sedimentation in the Cambrian period appears to have followed 

 the general laws that govern deposition in periods of comparative 

 freedom from great deforming movements, and hence of progressive 

 base-leveling. Most of the known Cambrian sediments were 

 deposited in the sea, and their area may be regarded as a rough 

 measure of the area of the sea at that time. Sedimentation was 

 probably faster in the early stages of the period when the land- 



