STRATIGRAPHIC TAXONOMY 595 



The black shale faunas, which are largely made up of pelagic species 

 and are neither very common nor varied, may be expected to give the 

 most trouble. Thus it required close comparison to prove that the 

 Maquoketa Eichmondian fauna is really much younger than the Utica 

 fauna, or that the faunas of the early Trenton Martinsburg shale and 

 of the middle Blount (pre-Lowville) Normanskill and Athens shales 

 are much older. Of course, we know now how to distinguish them 

 almost at a glance, but the belief that they are all of the same age — ^that 

 is, Utica — prevailed for a long time. And we are not past the danger 

 of confusing certain faunal facies of the Moorefield and Fayetteville 

 shales, two Tennessean formations, with the Pottsvillian Caney shale 

 fauna. The differences between the Genesee and Chattanoogan faunas, 

 also, are not so clearly marked that one may distinguish them offhand. 

 Finally, there is no satisfactory warrant for the claim that because 

 certain black calcareous shales at the base of the Eomney in Maryland 

 and adjoining States contain pelagic or pseudopelagic Marcellus species 

 like Styliolina fissureUa and Strophalosia truncata the beds are of the 

 Marcellus age. These species doubtless existed before and after the 

 Marcellus, and in the case mentioned they, together with surviving 

 Oriskany ostracods, may well have mingled with an Onondaga fauna in 

 Maryland. 



After all, we should treat these slowly modifying black shale faunas 

 just as we do the recurrent littoral faunas — that is, we should estimate 

 their respective time values according to the evidence of the few new 

 forms that came in with each invasion, and which after well tried ex- 

 perience are set aside as reliable guide fossils. Frequently but one or two' 

 such species may serve our purpose very well, while general "matching'" 

 of full lists often leads to no definite result. 



CLASSIFICATION BY DIASTROPHIC MOVEMENTS 



Maximum, major, and minor movements. — All diastrophic movements 

 and processes have ever been characterized by periods of activity alter- 

 nating with periods of relative quiescence. Periodicity, then, is a funda- 

 mental factor of geologic history. Further, all diastrophic processes arc 

 rhythmic in operation and recurrence, because they are occasioned by the 

 necessarily rhythmic action of terrestrial forces. In some the meter is 

 long, in others relatively short; in some the effects are pronounced and 

 relatively impulsive, in others gentle and gradual in development. Dias- 

 trophic movements thus offer a basis for classification. 



There were maximum movements or "revolutions." These were 

 marked bv stronsj crustal deformation with decided horizontal move- 



