CAKBOOTEEKOUS SYSTEM. 269 



and in some other respects this horizon partakes strongly 

 of the characters of the Upper coal-measnres. 



The average stratigraphical thickness of the Middle coal 

 formation is probably not far from two hundred feet, although 

 the maximum thickness is between three hundred and four 

 hundred feet. The thickness of the strata intervening between 

 the three principal coal horizons is variable in different 

 localities. The middle member is the thickest, the upper and 

 lower divisions probably averaging about the same thickness 

 or about sixty feet. The middle division on the Middle 

 Raccoon river, in Dallas county, near Redfield, where it has 

 an exaggerated development not observed elsewhere, attains 

 a thickness of probably not less than one hundred and fifty 

 feet. At some localities the lower division may reach one 

 hundred feet in thickness ; but the greatest observed develop- 

 ment of the upper division does not exceed eighty feet. 



The Middle coal-measures are traversed by more or less 

 regular wave-like undulations, which have a parellelism that 

 would be difficult to account for in any other way than that 

 they are the result of widespread, through greatly modified 

 disturbance. The conclusion is further substantiated by the 

 discovery of similar undulations in the strata of the Upper 

 coal-measures in southwestern Iowa, which have a north- 

 easterly and southwesterly trend, and are apparently in 

 continuation of those that cross the Middle coal-measures. 

 In a field so narrow as that occupied by the outcropping 

 strata of the latter formation, and where there are no 

 established base lines to facilitate and systematize explora- 

 tions, it is extremely difficult to distinguish the general from 

 the subordinate undulatious, which latter may have a 

 direction more or less divergent from that of the main folds ; 

 and it was not until the examinations made by Dr. White, in 

 southwestern Iow r a, brought to light the existence of identical 

 phenomena that we could, with any degree approaching 

 accuracy, determine the general trend of these undulations. 

 That these slight folds were also communicated to the Lower 

 coal-measures there can be no doubt; but as yet they have 



