307 



calcareous rocks, and seem to be most typical of calcilutytes or 

 fine calcarenyt-es. Such rocks, composed largely of the finest 

 lime-mud, accumulate in shallow water or in part even above 

 the normal level of the sea. In form they probably constitute 

 a sort of mud flat delta, the mud being brought and spread 

 out in part at least by streams, which must have derived it 

 from the erosion of earlier limestones. Mud cracks or 

 desiccation fissures testify to the shallow water in which these 

 sediments were accumulating. On exposure partial hardening 

 permits the formation of a superficial crust, which may 

 subsequently breapk or become deformed by the sliding of the 

 entire mass seaward." Under this heading Grabau also 

 includes, ''the edgewise conglomerates of many limestone 

 formations," in which the fragments sometimes occur on end, 

 and also the ''Suh aquatic, Gliding-deformations," which, it 

 it said, sometimes take place on a gently sloping sea or lake 

 bottom, by which the sediments become broken up, contorted, 

 and breociated.(i6) 



ENTEROLITHIC STRUCTURE. 



These terms have been used respectively by Hahnd*?) and 

 Grabau (is^ for such deformations as occur within a rock that 

 are caused by chemical changes that produce an increase or 

 lessening of volume. Thus when anhydrite by hydration 

 becomes changed to gypsum there is a great increase of volume 

 which, in expanding, distorts the layers; or, conversely, in 

 the dehydration of gypsum there is a corresponding distortion 

 by contraction. Similar effects have been produced by the 

 recrystallization of the salt deposits in the Zechstein of North 

 Germany. The dolomitization of limestone is often attended 

 by local distortion, as, also, the changes induced by the 

 phosphoratization of beds of limestone. According to Grabau, 

 "The important distinction between enterolithic and other 

 deformations, such as folding under lateral pressure, or 

 gliding in a given direction, lies in the fact that the 

 enterolithic structure folds in all directions — is apolar, or 

 multipolar, instead of unipolar." 



DESICCATION CONGLOMERATES. 



In the Ohio coal-measures there are a number of 

 thin limestones, usually magnesian with additions of silica 



(16) Grabau, op. cit., pp. 779-785, with numerous figures. 



(17) Hahn, ''Untermeerische Gleitungen bei Trenton Falls 

 (Nord Amerika) und ihr Verhaltniss zu ahnlichen Storungsbil- 

 dern," Neues Jahrbuch fiir Mineralogie, u.s.w. Beilage Bd. xxxvi. 

 (1912), pp. 1-41, taf. i.-iii. 



(18) Grabau, op. cit., p. 756, et. seq. 



