HELDERBERG CROUP. 



25 



Monroe County, the southeastern part of Wayne County, and the 

 same part of Lenawee County. This triangular segment is the 

 northern terminus of a large body of the formation, which covers 

 the northwestern part of Ohio,"and is continued through the State 

 of Indiana. It is on the west side of the Cincinnati axis of eleva- 

 tion ; the beds are not disturbed by the uplift in their horizontal 

 position. 



In Monroe County the beds of nearly all the creeks present 

 exposures of the ledges ; in Lenawee and Wayne counties, the in- 

 cumbent drift deposits are much heavier and the rock is not so 

 often exposed. The surface of all the rock beds of this region 

 which lie on top is worn smooth and scratched by drift marks. 



The lower division of the Helderberg series, identical with the 

 strata forming the base of Mackinac Island, and with the water- 

 lime group of the Ohio reports, has a much greater surface exten- 

 sion in the district under consideration than the upper division 

 corresponding with the limestones of Sandusky. This upper 

 division is well uncovered in the quarries of Trenton village in 

 Wayne County, and likewise in the quarries on Macon Creek, 

 formerly owned by Judge Christiancy. The surface layers of the 

 Trenton quarries are only covered by a thin coating of loamy 

 drift. They are limestones of light color, segregated in thin beds, 

 with uneven rough surface, containing an abundance of fossils, 

 particularly Crinoid stems, Bryozoa, Cyathophyllum cornicula, 

 various Favosites, Atrypa reticularis, Spirifer gregarius, Spirifer 

 acuminatus, etc. The thickness of the beds is about six feet ; 

 they are used for lime-burning, and yield a white, quick-slaking 

 lime of very good quality. The beds of the quarry have a very 

 perceptible dip west by southwest, which is evidently caused by 

 a local undulation of the strata. The direction of the dip of the 

 formation at large can not be learned from the measurement and 

 examination of single localities; it is generally hypothetically 

 deduced from the distribution and sequence of the rock beds over 

 areas of large extent. The next lower beds in the quarry are a 

 compact gray, crystalline limestone, in beds from eight inches to 

 two feet in thickness, and in all amounting to about eight feet, 

 which represent the principal quarry rock. They are rich in fossils ; 

 the species which I collected there are: Cyathophyllum Hallii, 

 Cyathoph. cornicula, Zaphrentis prolifica, Zaphrentis gigantea, Fa- 



