VEGETATION OF CONGLOMERATE ROCKS 383 



ing from the rock pores is collected. A few places are almost 

 constantly wet, due to a gradual seepage of water from the in- 

 terior of the rock. 



Direction of slope is important because of differences in ex- 

 posure to sunlight resulting in differences in evaporation (figs. 

 4 and 5). The effect is indicated in the progress of the plant 

 successions and the character of the final closed associations 

 found on different slopes. The texture of the rock material 

 determines the volume of water which may be held in the pores 

 of the rock, and the rapidity with which water may enter from 

 the outside. 



Measurements of the water content of small pieces of the 

 conglomerate from the surface of the rock mass showed an aver- 

 age of 4.45 grams water to 100 grams dry weight of rock. 



Rock weathering. The character of the weathering of these 

 Pleistocene conglomerates — slow surface decomposition without 

 sapping — is probably the most important factor in determining 

 the nature and completeness of the marked plant succession 

 on these rocks. It is chiefly because of this physical character- 

 istic of the habitat that the very marked plant communities 

 are developed. 



Soil. As a result of rock weathering and plant decay there 

 is accumulated in the irregularities of the surface, a very black 

 sandy humus. This soil is strongly calcareous, due to the pres- 

 ence in it of numerous minute grains of limestone produced by 

 the disintegration of the conglomerate upon decomposition of 

 its cement. The soil is extremely light, fine grained, and spongy 

 when wet, and does not bake on drying. The average capillary 

 water content as determined from a number of tests is 62.26 

 grams water to 100 grams of air dry soil. 



Isolation. Isolation of any area is an ecological factor be- 

 cause of the effect on plant distribution. Outcrops or groups 

 of outcrops of conglomerate rocks are frequently several miles 

 apart. The intervening space is forest, cultivated, or waste 

 land, usually having little in common \vith the rock habitat. 



Outcrops of the limestone and shale bed-rock of the region 

 are seen (1) along stream banks, (2) in ravines, (3) in quarries, 



