582 A. W. GRABAtI TYPES OF SEDIMENTARY OVERLAP 



Lake Superior sandstone. — This formation is generally referred to the 

 Upper Cambric and correlated with the Potsdam of New York and the 

 Saint Croix of the upper Mississippi valley. From a considt nation of the 

 facts furnished by the preceding sections, the progressive advance of the 

 Cambric sea over the North American continent has become apparent. 

 The advance was comparatively gradual, progressing through most of 

 Cambric time and not reaching the upper Mississippi valley until the end 

 of that period. It is therefore most likely that the basal beds of the Lake 

 Superior region mark a higher level than those of the Saint Croix area, 

 and their correspondence to the. lower Magnesian series is not improbable. 

 In fact, from their position it seems that they are more readily referable 

 to the Lower Ordovicic than to the Upper Cambric. The section of this 

 region is, however, complicated by the retreat and readvance of the Ordo- 

 vicic sea, of which the Saint Peter sandstone is the record. This will 

 be more fully discussed under another section of this paper, and there- 

 fore the consideration of the equivalency of the basal sandstone of the 

 Lake Superior region is deferred. A few local sections, however, may 

 be added here, to show that in places at least this sandstone is much 

 higher even than basal Ordovicic. 



Encampment d'Ours.* — On this island in the south channel (Lake 

 Huron) the base of the section is formed by the quartzites and slates of 

 the Huronian series, upon which rest unconformably 100 feet or more 

 of light colored soft, sometimes conglomeratic, sandstone. This is suc- 

 ceeded conformably by 60 feet of shales and limestones. The lower beds 

 of this series are "prevalently arenaceo-calcareous shales of a dusky green 

 or bluish color." They contain the following species: 



Species of the lower Bed Range Elsewhere 



Camarotoechia plena Hall Chazy. 



Rafinesquina alternata (Conrad) small var. Chazy to Richmond. 



Cyrtodonta huronensis Bill Stones River to Trenton. 



C. subtruncata 



Vanuxemia inconstans Bill Black River to Trenton. 



Matheria tener Bill Trenton. 



Liospira eugenia (Bill.) Black River. 



Orthoceras multicameratum Emmons Stones River to middle Trenton. 



O. granulosum Rominger 



Stictopora ramosa Hall Stones River. 



Callopora ramosa (D'Orbigny) Lorraine. 



Columnaria cystoceras, etcetera. 



* Rominger : Report on Paleozoic rocks of Upper peninsula of Michigan. Michigan 

 Geological Survey, vol. i, pt. Ill, 1873, p. 64. 



