36 COLORADO COLLEGE STUDIES. 



A modification of the arch-cave is seen where a vein 

 of water reaches an outcrop of the fjypsum and of the up- 

 per part of the Flower-pot shales in some ravine and, trick- 

 ling down over the outcrop of the gypseous clays, causes 

 the latter to soften and fall away in more or less vertical 

 sections, exxavating a clay-walled room with a gypsum 

 roof. This may be regarded as a special case of head- 

 water erosion, where the excavating done by the streamlet 

 follows the latter back into a yielding terrane beneath one 

 that is more resistant, and in so doing changes its work 

 from that of trenching to that of undermining. A good 

 example of a cave of this sort is seen in what may be called 

 the Green room, a cave in the west bluff of Bear creek, 

 some distance below the Natural bridge and in the imme- 

 diate vicinity of an older, partly fallen cave formerly much 

 visited by picnicking parties from Sun City. It consists of 

 a single spacious room partly walled up in front by a ridge 

 of talus from which it is necessary to descend into it as into 

 a cellar, and is called "the Green room" in allusion to the 

 fact that its roof is largely incrusted with a beautiful pale- 

 green layer of stalactitic gypsum studded with capitate pro- 

 tuberances like miniature cauliflowers. The latter, formed 

 by the evaporation of dripping water, are more stipitate 

 and have a lighter and more open structure than the large 

 cauliflower-topped masses forming in the pools of the Com- 

 anche cave. The color of this roof-crust is only superficial, 

 however, being due to conditions that favor the culture of 

 an undetermined alga, supposed to be one of the Cyano- 

 phyceas. The roof of the Green room is traversed by a 

 channel whose sinuous course marks the former continua- 

 tion of the vein that has produced the cave and is still con- 

 tinuing it inward. 



An interesting topographic feature of the Medicine 

 Lodge gypsum is its natural bridges. These are merely 

 remnants of caves that have for the most part fallen in. 

 Sometimes, as near Havard creek, in Barber county, Kan- 



