STATE GEOLOGIST. 61 



Tho well characterized limestones of the Upper Ilelderberg 

 Group, to the thickness of 250 feet, exist in a confusedly brec- 

 ciated condition. 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 vibrations and 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- 

 ments. 



This is the general physical character of the mass; but in. 

 many places the original lines of stratification can be traced, 

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

 various angles and in all directions, sometimes exhibiting 

 abrupt flexures, and not unfrequently a complete downthrow of 

 15 or 20 feet. These phenomena were particularly noticed at 

 the cliff known as Robinson's Folly. 



In the highest part of the island, back ol Old Fort Holmes, 

 the formation is much less brecciated, and exhibits an oolitic 

 character, as first observed in the township of Bedford, in 

 Monroe county. The principal part of Eound and Bois Blanc 

 Islands is composed of the brecciated mass. It forms the 

 promontory west of Mackinac, which, on the north side, sinks 

 abruptly to the low outcrop of the Onondaga Salt Group, 

 stretching across from the Hare's Back to Little Pt. au Chene. 

 It is seen again in the vicinity of Old Mackinac, but it evidently 

 diminishes in thickness toward the south. 



The elevated limestone region constituting the northern por- 

 tion of the peninsula, consists of the higher members of the 

 Upper Helderberg Group, which gradually subsides toward the 

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

 as can be judged, sinks beneath the shaly limestones of the 

 Hamilton Group. The strike of the formation determines tho 

 trend of the coast of Lake Huron, although the limestone bar- 

 riers to the lake are generally, at the present day, situated 

 some distance back from the immediate shore. A few miles 

 north-west of Adam's Point, at Crawford's marble quarry, the 



