CHAPTER IX. 



CARBONIFEROUS LIMESTONE. 



The rock beds succeeding the Waverly group indicate impor- 

 tant changes in the condition of things caused by the altered 

 nature of the ocean sediments. The sandy material so much pre- 

 vailing toward the end of the Waverly period begins to vanish ; 

 in place of arenaceous ledges interlaminated with the still con- 

 tinued shaly sediments, we find beds of limestone, and gradually 

 the shales become narrower subordinate seams, until the upper part 

 of the series is exclusively a limestone formation. The upper 

 limestones resemble, in rock character and by their fossils, the 

 upper division of the subcarboniferous limestones of the Mississippi 

 valley ; the loAver part of the formation, which, locally, incloses 

 heavy gypsum beds and attains a considerable thickness, is indu- 

 bitably the equivalent of the inferior division of the western sub- 

 carboniferous limestones, but can not be exactly parallelized with 

 a certain horizon of that series. 



The lower gypsiferous part of the group has been described by 

 Prof. A. Winchell under the name of Michigan salt group. I have 

 previously stated that the salt brines of Michigan are derived from 

 the subjacent Waverly group. Gypsum and salt are frequently found 

 associated in other parts of the world, which circumstance prob- 

 ably induced Mr. Winchell to locate the salt in this higher rock 

 series ; but the fact of the occurrence of the salt brine in a lower 

 position cannot be denied on the ground of mere theoretical specu- 

 lation. The carboniferous limestone series is very unequally de- 

 veloped [on the peninsula ; in some parts we find it scarcely repre- 

 sented, while in others the upper calcareous division is well de- 

 veloped, but not the lower gypsiferous beds, which seem to be of 

 restricted local extent. The lowest beds of the group can be ob- 



