VEGETATION OF CONGLOMERATE ROCKS 381 



the rock varies from a coarse conglomerate to a sandstone with 

 few pebbles. Within the same rock mass, the gravel varies 

 greatly in size, from particles the size of coarse sand grains to 

 bowlders several inches in diameter. 



The larger bowlders are embedded in a matrix of smaller 

 particles. The rock is extremely porous. The weathered sur- 

 face is very irregular due to numerous projecting bowlders, 

 and to small depressions or pockets formed when bowlders 

 weather out. 



Comparison ivith bed-rock of region. The conglomerates of 

 the Cincinnati region everyw^here tend to be more massive than 

 the bed-rock. Because of this there is a difference in the man- 

 ner, possibly also in the rate of weathering of the two types of 

 rock. Weathering of the conglomerates is due to the action 

 of surface agencies and ground water, resulting in the gradual 

 reduction of surface, wdth the accompanying formation of soil 

 on the rock surface, and the occasional loosening of surface 

 bowlders. In the weathering of bed-rock, there is, in addition 

 to the above factors, a pronounced sapping of limestone layers 

 due to the rapid disintegration of the interbedded shales. This 

 results in sudden and relatively great changes in the surface 

 of the rock exposed, and yields instead of fine soil, numbers of 

 large slabs of limestone which are relatively soon covered with 

 soil from the surrounding slopes. Decomposition is more ef- 

 fective in the weathering of the conglomerates than is disin- 

 tegration. In the case of the bed-rock the reverse is true. The 

 hillside around the conglomerate rock is frequently reduced to 

 a fairly gentle slope from which the rock mass projects. For 

 this reason, the conglomerates usually project as huge masses 

 from the hillside (fig. 1). 



ECOLOGICAL FACTORS 



Moisture, Except during and for a short time after rains, 

 much of the surface of these rocks is dry. Some water is held 

 for a time in the surface pockets. The accumulation of soil 

 in the pockets, and the deep moss mat are important in lessen- 

 ing run-off. The irregularities of the surface and the porous 



THE PLANT WORLD, VOL. 20, NO. 12 



