ART. 14 MICHIGAN TRAVEESE GROUP POHL 23 



are restricted to the lower unit: Several undescribed species of Acti- 

 nopteria, Ja/neia species, Paracyclas species, and Conocardiwii em- 

 metense Winchell. Of the gastropods the only remarkable forms 

 are three species of Pleurotomaria (?), P. cavumhilicata Winchell, 

 P. emmefensis Winchell, and P. parvispira Winchell, known to be 

 limited to the Charlevoix ; and a septate form of Euomphalus found 

 low in the Petoskey, Cephalopods are present but not commonly so, 

 and have received little or no attention. 



Of the trilobites, species of Phacops, Proetus, and Asteropyge 

 occur in both the Gravel Point and Petoskey. These have not so 

 far been studied. Ostracods of the Moorea and BeyricMlina types 

 are found in the Gravel Point and are abundant in some beds of the 

 Charlevoix but again hQ,ve not been differentiated nor described. 



Occasional fish plates of the Dinichthys and Ptyctodus types are 

 found in isolated occurence, but their rarity precludes any strati- 

 graphical usefulness. 



REGIONAL AND LOCAL STRUCTURE 



Much has already been said concerning the varied and complex 

 structure of the region bordering Lake Michigan. With the addi- 

 tional evidence now in hand several new features appear to further 

 complicate any intrepretation of these phenomena. 



Deformation versy^ reef origin. — It appears unquestionable that the 

 miniature closed anticlines and synclines which are so abundant at 

 all exposures of the Traverse beds in western Michigan are due to 

 an entirely local and indigenous cause. The positioas of numerous, 

 compact Prismatophyllutn and Stronnatopora reefs are seen to be 

 geographically and stratigraphically fortuitous, and upon consolida- 

 tion the strata would naturally conform to the irregularities caused by 

 these more compact masses. Reefs of large dimensions are, however, 

 entirely absent, and for the explanation of the larger domes, anti- 

 clines, basins, and synclines we must look to other causes. One un- 

 doubted emergence and probable deformation on a slight scale has 

 already been shown to have occurred within the Traverse. The oscil- 

 lating character of the central basin portion of the Devonian province 

 has just begun to be appreciated. The regional dip has been remarked 

 before as trending irregularly southeastward. Faunal discrepancy 

 between the western and eastern areas of Traverse outcrop is becoming 

 more and more evident as the associations become increasingly under- 

 stood. It is rather diflScult to account for this faunal distinction on 

 any other basis than the hypothecation of at least an intermittent 

 land barrier separating the two regions. This supposition requires a 

 remodification of structural attitudes to account for the present day 

 regional structure. 



