408 J. E. Hyde — Desiccation Conglomerates. 



along- horizontal lines and as a result some of the pebbles look 

 quite old. This seems to be due to the probable fact that at 

 the time of formation the dried mud in the pebbles, while 

 hard enough to withstand considerable rolling, was still soft 

 enough to yield readily to the agents of weathering and erosion 

 if exposed for a moderate period of time. The surface of the 

 matrix after consolidation appears to have been exposed to the 

 atmosphere for a short period, as quartz grains and small shells 

 are often found standing in relief. 



The succession of events in the formation of the bed ap- 

 pears to have been about as follows: A Jbocly of water, , 

 probably of no considerable depth, existed, in -which lime t 

 muds had accumulated to a slight thickness. On evaporation 

 all ostracods and Spirorbis shells would be laid down "on the 

 surface of the mud. Such a layer of shells is sometimes seen 

 on well-preserved mud-cracked surfaces in these limestones 

 and a few such shells are found on the surface of some of the 

 blocks in this layer but never to any extent, as they were prob- 

 ably subsequently rubbed off. After the complete evaporation 

 and cracking of the lime surface, it is necessary to suppose 

 that there was a submergence in order to account for the 

 movement, breaking and rolling of the pieces and the for- 

 mation of the matrix of small fragments and shells in which the 

 pebbles all rest. The presence in the matrix of a considerable 

 amount of quartz sand which is absent from the limestone of 

 the pebbles and the underlying bed is best accounted for in 

 this way, the size and angularity of the grains being contrary 

 to the idea of wind-blown sand. Whether the animal remains 

 in the matrix were rubbed oif the pebbles ( they are numerous 

 both in the pebbles and in the underlying limestone ) or 

 whether they lived in the water which caused the breaking up, 

 is not known. This point is mentioned because they do not 

 occur in the overlying dark shale. Following the breaking 

 up of the sun-cracked blocks, there must have been another 

 period in which the surface was dry and during which it hard- 

 ened into the form now seen. Proof of this is seen in the 

 weathered surface of the matrix with its quartz grains and 

 ostracod shells in relief and in the perfectly sharp line of con- 

 tact between the conglomerate and the overlying shale, which 

 can only be accounted for on the assumption that the former 

 was completely formed and hardened as now seen before the 

 submergence which brought the deposition of the shale. 



If, after the conglomerate was completely formed, the depo- 

 sition of limestone had been resumed instead of a soft shale, 

 the result would have been a typical intraformational con- 

 glomerate of a thinner type, in which the structure would prob- 

 ably have been so obscured that a detailed study would have 

 been impossible or only possible with a great amount of labor. 



Stratigraphic Laboratory, Columbia University. 



