THE PERMIAN SYSTEM IN KANSAS. 33 



With a strength due to the character of its consoHda- 

 tion and to its massiveness, the Medicine Lodge gypsum 

 combines the weakness due to its solubility. Owing to 

 that strength, it is among its fellow-terranes a piece dc resist- 

 ance, and is, like the Loup Fork sandstone, responsible for 

 some of the highest bluffs and deepest canyons seen in the 

 central portion of the Plains. Owing to that weakness, it 

 is essentially a cave formation. Indeed the gypsum in some 

 locaHties is fairly honeycombed with earth-lilled or empty 

 galleries and spaces. 



Its caves are formed in two ways, and may accordingly 

 be classified as rift-caves and arch-caves. 



The gypsum is parted into large blocks by vertical 

 master-joints which often persist for long distances, and 

 which become the conduits of meteoric waters. Enlarged 

 at first by solution and later perhaps in part by mechanical 

 erosion, the joint-tissure becomes a narrow and at length 

 somewhat wider gallery, or rift-cave. It may remain open 

 or, if it come to be traversed by an intermittent or variable 

 current carrying sediment, it may become wholly or partly 

 filled with the latter, and so be either a potential cave or an 

 actual one with earthen floor, as the case may be. 



In forming the arch-caves, water descending through 

 joints or other crevices in the gypsum, is arrested at the 

 summit of the Flower-pot shales and finds its way as a vein 

 along the base of the gypsum, excavating the lower part of 

 the latter and sometimes also a portion of the underlying 

 gypsiferous clay by a corrasion in which solution plays the 

 leading part. In the case of the clay, its fine particles, set 

 free by the solution of the associated gypsum, may be car- 

 ried off b}- even a feeble current, the readiness with which 

 these and other red clay sediments of the Cimarron series 

 are held in mechanical suspension in water being frequently 

 attested on the Cimarron outcrop by the slow settling of 

 these sediments in rain-pools. 



The rift-cave is doubly typified in what may be called 



