FEATURES OF FORT DODGE GYPSUM 



597 



Professor Thomas suggests that the domes have been caused by 

 the crystals of gypsum in the upper layers absorbing an excess of 

 water with a consequent expansion and a heaving up of the lay- 

 ers into domes. In some cases, as is shown in the center back- 

 ground of figures 202 and 203, the expansion has resulted in a 

 buckling of the layers. 



As to the age of these phenomena, the first and natural as- 

 sumption would be that they were very recent, later than the un- 



Fig. 204. — Channels of solution in the gypsum in Two Mile creeli valley, at 

 the same locality as the last two views. 



covering of the beds. This may indeed be the case but the gen- 

 eral condition of the beds — their buckling and tilting, their so- 

 lution channels and extreme weathering — seem to the writer to 

 point rather to a greater age. There are so many possibilities 

 for the time of formation of these features — postglacial, inter- 

 glacial and pregiacial — that it does not seem possible to decide 

 upon any one of them. 



Iowa Geological Survey, 

 Des Moines. 



