308 



"and clay, and apparently of both marine and fresli-water 

 origin. Mr. J. E. Hyde, of tho Stratigraphic Laboratory, 

 Columbia University, in describing theses limestones, says, 

 ''Layers of conglomerate, consisting of fragments of limestone 

 cemented in a limestone matrix, occur throughout the series. 

 Their thickness may vary from a fraction of an inch to two 

 or three feet. The fragments are usually small, from the 

 size of a pea or less up to an inch or two in diameter, and, 



rarely, four or six inches in diameter The origin of 



these limestones is an open question, but it is probable that 

 they were formed in restricted basins whiclPmay have had an 

 extent of many miles. The niunerous mud-cracked surfaces 

 and abrupt lithological changes show that the area was 

 subjected either to rapid oscillation and consequent successive 

 flooding and drainage, if they be considered of marine origin, 

 or to periods of desiccation if they be considered as lake 

 deposits." For this kind of broken bedding Hyde suggests 

 the name of Desiccation Conglomerates,^-^') and thinks that 

 they were formed somewhat under similar conditions to the 

 fresh marl lakes that occur at the present time throughout 

 much of the drift-covered area of North America. Hyde 

 says, ''The method of formation is briefly this: Extensive 

 area? of lime-mud may be exposed to the atmosphere either 

 by elevation above a body of water, due to land movements, 

 or by the evaporation of an enclosed body of water in which 

 such muds exist. On drying, the surface of these lime-mud 

 flats would become cracked and hardened, and if exposed a 

 sufficient length of time the cakes thus formed might become 

 hard enough to withstand more or less working over by waves 

 on resubmergence. If it should then be covered by succeed- 

 ing lime-muds, the layers would constitute a true intra- 

 formational conglomerate," (20) 



Walcott had, in 1896, noticed certain shore features on 

 Khode Island which might illustrate the formation of 

 desiccation breccias and conglomerates. He says, "The mode 

 of formation of a brecciated limestone conglomerate has often 

 puzzled me when studying Palaeozoic rocks. Geologists 

 frequently observe layers composed almost entirely of thin- 

 bedded, brecciated limestones, or of sandy or argillaceous 

 shales. In many cases the layers of breccia are not more than 

 an inch or two in thickness and occur between layers formed 

 of undisturbed sediment. The breccia is often composed of 

 angular fragments varying from a quarter of an inch to half 



(19) See Hyde, •'Desiccation Conglomerates in the C'oal- 

 measures Limestones of Ohio," Am. Jour, of Science, Fourth Ser., 

 vol. XXV., 1908, pp. 400-408. 



(20) Loc. cit., p. 401. 



