ORIGIN, MODIFICATION, AND AGE. 239 



of former salt beds which rested over the gypsum and were much thinner 

 than the beds farther south. The salt water percolated downward to 

 rise again in the salt springs of that region. 



The swamp deposits of earthy gypsum are clearly secondary and in 

 age are geologicall}'- very recent. The deposits occur in low, swampy 

 ground, and strong springs of gypsum water occur in nearly all of them. 

 At the same level, or from 10 to 20 feet below the dirt, is a stratum of 

 solid gypsum, while near most of these deposits no gypsum is found above. 

 Near the bottom of the Rhodes deposit were found an Indian spearhead 

 and recent shells of genera* Planorbis and Physa, while from the Dillon 

 deposit the supposed buffalo bones were obtained. 



Gypsum in an earthy form is deposited at the present time in dry 

 weather to the extent of a half inch in a few days by the evaporatien of 

 running water near these places. Where the gypsum water of the springs 

 in these beds is evaporated there remains ji crust of gray earthy gypsum 

 resembling very closely the dirt. Sand, clay, and lime also occur in the 

 deposit mingled with some organic material. 



Part of the deposit in the form of silica, lime, clay, and gypsum has 

 doubtless been washed from higher levels, but the greater portion has its 

 source in the underlying gypsum rock. The circulating water has dis- 

 solved part of this ledge and carried it upward in the springs to the 

 surface of the swamp, where the mineral was precipitated through evap- 

 oration, aided by the action of the organic matter of growing vegetation. 

 The deposit accumulated through a comparatively short period of time 

 and is even now slowly increasing in many places. 



SOUTHERN AREA. 



At a later period in the geological history of Kansas another extensive 

 gypsum area was formed in the southern part of the State, Oklahoma, 

 and Texas. This is found in the red beds which probably mark the 

 close of Permian time. The ripple-marked sandstones associated with 

 the Medicine Lodge gypsum indicate that the deposition was in a shal- 

 low bay, and its great thickness would necessitate a slow depression of 

 the floor of that bay. The uniformity in character of the whole deposit 

 shows that similar conditions were present over a large area. The ab- 

 sence of clay seams indicates that very little, if any, sediment was car- 

 ried into the bay at that time, though the thick deposits of red shales 

 below and above the g}'psum show the sedimentation was great and only 

 interrupted for a time, possibly the interruption being the result of cli- 

 matic change. The dip is southward, and probably the gulf opened to 

 the south or southwest, as the deposit thickens in that direction. 



* Discovered by Dr S. Z. Sharp. 



