INTRAFORMATIONAL PEBBLES IN RICHMOND GROUP 303 



In the case of sea deposits, the material filling mud-cracks might 

 include coarser-grained deposits or organic material, either entire 

 or fragmental. Now, it is the very frequency of the supposed mud- 

 cracks with the absence of the concurrent phenomena here indicated 

 which throws doubt on their interpretation. In such "mud- 

 cracks" as have been observed hitherto, the material filling the 

 cracks is essentially the same material as that forming the lateral 

 walls of the crack, and no fossil or fragmental material has ever 

 been found in a position suggesting that it had been dropped into 

 the crack, or had been washed into it. 



On the contrary, in many cases it has appeared possible that the 

 cracking could have occurred subsequent to the deposition of the 

 overlying strata, in fact long subsequent to the latter, and may not 

 be due to the drying effects of the air along a seashore, but to shrink- 

 age of strata deposited in much deeper waters. Mud deposits 

 in quiet waters have been known to crack without exposure to the 

 air, although the observed cracks have always been of too small 

 magnitude to suggest mud-cracks. Shrinkage, however, may have 

 occurred also long subsequent to the deposition of the overlying 

 strata, during a period of elevation of the entire mass of marine 

 deposits. The gradual filling of the cracks might have been accom- 

 plished by slowly circulating waters while the shrinking material 

 still was comparatively soft. While the method of filling of these 

 cracks may vary in different cases, the possibility of their origin 

 from shrinkage long after the deposition of the strata in which they 

 occur should be considered. If anyone has any evidence of the 

 presence of mud-cracks in Cincinnatian rocks which unquestion- 

 ably are due to elevation of mud flats above water-level before the 

 deposition of the immediately overlying strata, this evidence should 

 be published in detail. 



Until the presence of raindrop impressions and of mud-cracks 

 due to exposure of mud-flats to the open air before the deposition 

 of the overlying strata has been proved unequivocally, it is not so 

 certain that ripple-marks indicate shallow-water conditions. They 

 may have been formed a considerable distance below sea-level, at 

 least sufficiently far not to necessitate the immediate presence of a 

 shore line. 



