J. E. Hyde — Desiccation Conglomerates. 401 



worn from the ridges was deposited in the intervening depres- 

 sions beneath the sea."* The presence of occasional large 

 bowlders in the bedded strata either above or below the con- 

 glomerate bed is explained by dropping from floating ice. 



For all the various types of conglomerate which can be in- 

 cluded under the term intra-formational conglomerate, it is very 

 probable that a number of different explanations are necessary. 

 The thin brecciated layers only a few inches in thickness com- 

 posed of relatively small fragments of limestone have certainly, 

 in some instances, been formed in a manner at least different in 

 detail from that outlined above. Walcott, from observations 

 made on the shore of Rhode Island, suggests a method by which 

 the formation of such breccias may take place. A thin crust 

 of fine sand and mud which had had sufficient time to dry and 

 harden after ebb tide, was broken rip by the following flood 

 into small angular flattened fragments which were heaped up 

 in places several inches deep and in one case covered by sand. 

 He suggests that if this should occur on a sinking shore line 

 conglomerate layers like those often seen in sedimentary rocks 

 would result. f 



The purpose of the present paper is the consideration of one 

 of these possible types, which, although apparently not of com- 

 mon occurrence is, nevertheless, distinct enough to be worthy of 

 notice. It is possible that close observation may show it to be 

 of more common occurrence than has been suspected. While 

 in a general way formed in a manner quite similar to that 

 suggested by Walcott as outlined in the last paragraph, it is 

 peculiar in its details. The method of formation is briefly this : 

 Extensive areas of lime mud may be exposed to the atmosphere 

 either by elevation above a body of water due to land move- 

 ment 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 succeeding 

 lime muds, the layer would constitute a true intra-formational 

 conglomerate. A stratum so formed will probably be of no 

 great thickness unless fragments from a considerable area should 

 be washed into a limited one. 



There are a number of conglomerates in the so-called "fresh- 

 water" limestones of the Ohio Coal-measures which seem to 

 have had this origin, but in only one instance is it so Avell shown 

 that a detailed description is possible. In order to present this 

 one example clearly, a brief summary of the little that is known 

 of this type of formation is given. If further investigation 



*Bull. Geol. Soc. Am., vol. v, p. 197. Bull. 134 U. S. G. S., p. 89. 

 fBull. TJ. S. G. S., No. 134, p. 40. 



