ART. 14 MICHIGAN TRAVEESE GROUP POHL 17 



sinuate, usually horizontally compressed fucoids having an average diameter 

 of one-half inch. Passing to the south, some 30 feet of thin bedded, finely crys- 

 talline dolomites interbedded with irregular, nodular bands of chert are exposed, 

 due to the southerly dip, in several low bluffs. Near the top of these beds (the 

 " Cherty Beds" of Winchell), are several 1-foot bands of coarse dolomite 

 containing several corals {Heliophifllum, Favosites hamiltoniae (?), and a 

 questionable Cladopora) and a large Atrypa. Near the southernmost exposure 

 some large " kettles," radially arranged authracolite concretions having as a 

 usual nucleus an arthrodiran plate, weathered from the immediately succeeding 

 Waverlian black shale were observed, but no black shale was seen in contact 

 with the underlying Petoskey beds. 



The sections as thus combined to give a complete succession for 

 the Traverse Bay district aggregate approximately 250 feet in ex- 

 posed beds. It is, of course, impossible to even hint at the thickness 

 of the unexposed portion of the Petoskey. It is hoped that further 

 search will bring to light the beds to fill this gap ; but for the present 

 it seems probable from attendant circumstances that these will not 

 extend the section very considerably. From a study of the physical 

 relations this volume of deposition is found to be unequally appor- 

 tioned between three divisions — 120 feet in the Gravel Point stage, 

 28 feet for the Qiarlevoix stage, and 100 feet for the Petoskey forma- 

 tion, which latter may be increased if the unknown beds are later 

 discovered. 



Physical criteria hea7'in(/ on the restriction of stratigraphic v/nits. — 

 It has been shown that throughout the thiclaiess of beds below the 

 " Blue Shale " there have been common recurrences of a certain 

 number of lithologic characteristics. To sum these up connectedly 

 there has been a singular absence of the coarser clastic materials, 

 especially the silicates, while there is a frequent alternation of fine, 

 gritless shales with the shaley and comj)act limestones. Fossiliferous 

 black shale partings are particularly abundant toward the middle of 

 the Gravel Point section, while toward the top fine, blue, clayshales 

 become increasingly apparent. A great majority of the sediments 

 are made up of the darker, compact limestones which in many cases 

 are strongly bituminous. Cryptocrystalline limestones with a litho- 

 graphic texture comprise much of the beds near the middle of the 

 series. Reef structures of limited vertical and lateral extent are one 

 of the commonest features throughout the Gravel Point. While 

 the distinction between lithologic layers is usually sharp there is not 

 the slightest suggestion of broadly unstable conditions within the 

 series. The vertical lithologic variation is to be explained rather on 

 the basis of differential, not too shallow water current sorting and the 

 tendency on the part of the sediments to fill in the depressions 

 between the comparatively rapidly developing PHsinatophyllum and 

 Btromatopora reefs. 

 61590—29 3 



