STATE GEOLOGIST. 01 



The well characterised Limestones of the Upper Helderberg 

 Group, to the thickness ol S50 feet, exist in i ooofusedJy 



The individual fragments of the mass are 

 angular and seem to have been but little moved from their 

 original places. It appears as if the whole formation had been 

 shattered by sudden vibrate as an 1 unequal uplifts, and after- 

 wards a thin calcareous mud poured over the broken mass, per- 

 colating through all the interstices, and re-cementing the frag- 

 ment 



This is the general physical character of the mass; hut in 

 many places the original lines of stratification can be traced, 

 and individual layers of the formation can be seen dipping- at 

 varions angles and in all directions, sometimes exhibiting- 

 abrupt liexures, and not unfrequcntly a complete downthrow of 

 et These phenomena were particularly noticed at 

 the cliff known as Robinson's Folly. 



In tin- 1. '. lit of the island, back ot Old Fort Ilolmes, 



rmation is much less brecciated, and exhibits an oolitic 



chars I observed in the township of Bedford, in 



Monroe county. The principal part of Round and Bois Blanc 



Island* sed of the brecciated mass. It forms the 



promontory west ot' Mackinac, which, on the north side, sinks 



abruptly to the low outcrop of the Onondaga Salt Group, 



from the Hare's Back to Little Ft. an Chene. 



kgain in the vicinity of Old Mackinac, but it evidently 



diminishes in thickness toward the south. 



The elevated lim egioa constituting the northern por- 



tion of tii'- peninsula, • <>f the higher members of the 



Upper Belderl i ip, which gradually subsides toward the 



south, and in the southern part of C county, as nearly 



as can be judged, sink tth the ahery lim* f the 



Hamill I up. The strike of the formation determines the 

 f Lake fcTur >n, although the li bar- 



rally, at the present daj . 



tk from the iinmediab miles 



north- :i;n iy, the 



