338 W. I. ROBINSON 



It may be noted that this sort of fold would be the most imperfect 

 "structure" possible for accumulating light liquids and gases, 

 because the pitch would be uniform from the point of inception of 

 folding to the outcrop and would be equal to, or greater than, the 

 dip of the series as a whole. 



Another explanation of the origin of these folds is that they are 

 the result of unequal subsidence of portions of the periphery of the 

 basin, but this explanation is less satisfactory in the case of the 

 Michigan folds because such subsidence would be expected to 

 produce faulting and folds of widely varying dimensions and 

 unequal distribution. 



An unequal distribution of soluble material such as rock salt near 

 the periphery and its removal by solution, or the unequal solution 

 of parts of a salt horizon, have been suggested to account for these 

 folds. Considering only that area in southeastern Michigan where 

 both the folding and the salt beds are best known, this explanation 

 seems most reasonable, but as the folds are also known from other 

 parts of the basin where the presence or absence of salt beds of 

 sufficient thickness to produce the subsidence effect has not been 

 proved, this, with the other explanations, must be only tentatively 

 considered until more data are at hand. Whichever of the above 

 hypotheses is favored, it will be noticed that the underlying cause 

 is a vertically acting force, although under the subsidence hypothesis 

 first mentioned, the force is supposed to be resolved into components 

 acting tangentially around the periphery. 



An examination and platting of well records already assembled 

 by Lane^ and Smith^ results in the following pertinent suggestions. 



1. The folds affect the latest rocks which are preserved — the 

 Mississippian and Pennsylvanian. 



2. There is an indication of gentle monoclines or terraces around 

 the central basin which suggests two periods of subsidence. 



3. These terraces separate the comparatively deep central area 

 from the shallower peripheral area, and thus are suggestive of two 

 distinct diastrophic areas. 



' Mich. Geol. Surv., Vol. V, 1881-93. 



^ Mich. Geol. Surv. Pub. 14, Geol. Series 11. 



