184 



like Betula lutea, Juglans cinerea, Tilia americana, and even 

 Tsuga canadensis, most of which are present in the ultimate 

 forest. 



The complexity of the surface geology of this state has been 

 referred to in a previous paper.* Most of the underlying rocks 

 are covered with sand, gravel, or clay; but in addition to the 

 trap rocks — basalt and diabase — the surface outcrops include 

 granites, gneisses, schists, quartzite, sandstone and shale, and 

 limestone. To what extent, it may be asked, do the successional 

 phenomena on these other types of rock correspond with, or 

 disagree from, succession on trap? In other words, how is 

 succession influenced by the physical or chemical nature of the 

 substratum. In the opinion of Cowlesf the most important 

 feature of a rock, as regards its effect on succession, is its sta- 

 bility, i. e., its degree of resistance to erosion. On stable uplands, 

 where erosion is slow, succession is likewise slow. On unstable 

 uplands, on the other hand, succession is rapid. Thus "in a 

 given region a shale area may be clothed with a mesophytic 

 forest, while a dolomite outcrop is still xerophytic, or a quartzite 

 is scarcely more than a naked hill." The influence of the physio- 

 graphic state of a region on the character of the vegetation is far 

 more important than either the physical or chemical nature of 

 the underlying rock. "The flora of a youthful topography in 

 limestone more closely resembles the flora of a similar stage in 

 sandstone than [that of] a young limestone topography resembles 

 [that of] an old limestone topography." In other words, "rock 

 as such, or even the soil which comes from it, is of less importance 

 in determining succession than are the aerial conditions, especially 

 exposure. And it is the stage in the topography that determines 

 the exposure." 



In view of Cowles's conclusions, which are based on the study 

 of succession in various parts of the eastern United States, it is 

 not surprising to find that in comparing successions on different 

 types of rock in this state the resemblances are much more pro- 



*Torreyai3: 109. 1913. 



t Cowles, H. C. The influence of underlying rocks on tiie character of the 

 vegetation. Bull.Amer. Bur. Geog. 11: 1-26./. i-io. 1901; Bot. Gaz. 31: 89, 

 90. 1901. 



