STRATIGRAPHIG TAXONOMY 619 



about equivalent to the Shady limestone, hence younger than the Hesse 

 sandstone, has not been determined. My tentative view inclines to the 

 second of these possibilities rather than the first and third. The Shady 

 limestone in that event would be younger. 



Acadian or middle Cambrian faunas and deposits. — Like the lower 

 Cambrian, the middle Cambrian series also attained its greatest devel- 

 opment in western North America. Walcott has described the sec- 

 tions in considerable detail and has gathered an amazingly rich and 

 varied fauna which is in course of study and publication. The fossil 

 treasures which have rewarded his unflagging zeal, especially the beau- 

 tifully preserved collections made in the vicinity of Mount Stephen in 

 British Columbia, tend to show that after all we know but a small 

 part of the life that teemed in the permanent oceanic basins and which 

 invaded the relatively occasional continental seas only when physical 

 conditions were favorable. Crustacean and other types are found in 

 the Mount Stephen formation whose origin was hitherto believed to be 

 much less ancient. Such discoveries show, as nothing else can, the danger 

 of relying implicitly on the general composition of a fauna in determining 

 the age of the beds containing it. For, if a fauna or any of its generic 

 or specific types can be shown to have existed at earlier dates than the 

 accepted, then it is perhaps even more — certainly no less — likely that 

 they continued to exist somewhere for long or shorter periods after their 

 apparent extinction in the standard section. It seems to me especially 

 unsafe to assume that a genus or family of vigorous organisms was uni- 

 versally exterminated at the close of an intrasystemic epoch or even 

 in the ordinarily longer intervals between most of the periods. Such 

 eifective faunal breaks probably occurred in a marked degree only at 

 the time of the diastrophic revolutions which define the eras of geologic 

 time. 



A case in point is brought out by the prevailing practice of referring 

 all beds containing Mesonacidce save Paradoxides, to the lower Cambrian. 

 If an Atlantic branch of the family could survive the presumably stressful 

 transition from the lower to the middle Cambrian, namely, by modifica- 

 tion to the Paradoxides f acies, why may not a Gulf of Mexico or a Pacific 

 type of the family have similarly maintained its existence? How may 

 we decide whether either did or did not? So far as I can see, the only 

 competent way is by means of purely diastrophic criteria. The present 

 practice rests solely on the assumption that Olenellus, using that term 

 in the broad old sense, is confined to lower Cambrian deposits. But is it ? 



I do not know how the question should be answered in the case of the 



