ECOLOGY OF NORTHERN MICHIGAN. 41 



swampy areas. If the imdrained conditions of these areas are caused by the 

 sinking of the coast, this habitat will tend to become destroyed by the 

 encroachment of the beach conditions. If, on the other hand, this encroach- 

 ment is relatively slow compared with other changes, the accumulation of 

 the inwash and organic debris will tend to convert such a swamp into a dryer 

 habitat and cause the present biota to be succeeded by a society adapted 

 to the new conditions. 



//. 2. North Slope of First Ridge. This slope on account of its relief 

 is well exposed both to the forces of weathering and of denudation and is 

 covered by a layer of soil that becomes thinner near the top of the ridge. 

 Above this soil there is a layer of humus and vegetable mould that tends 

 to conduct the water from the surface, thus greatly diminishing the effects 

 of denudation. (It also furnishes organic constituents to the soil). 



These conditions are very favorable for plant life, and the lower part of 

 the slope is covered by a dense forest with which is associated a large number 

 of animals. This biota is composed partly of forms whose principal dis- 

 tribution is in southeastern North America, and partly of those which 

 predominate to the north of the Great Lakes and in the bogs to the south. 

 On the thinner soil near the top of the ridge, those forms of the forest pre- 

 dominate that can live in the more open, poorer soil, and dryer conditions 

 of this habitat. 



The processes working on this slope tend to reduce the ridge toward 

 base level and to increase the depth of the soil. The accumulation of humus 

 retards the former process, while the latter proceeds more and more slowly 

 as the soil increases in thickness. Changes are thus taking place very slowly, 

 and the physiographic processes are evidently tending to approach an equili- 

 brium. Near the top of the ridge, owing to thinner layers of soil and humus, 

 the changes are more rapid but in the direction of the conditions that prevail 

 on the lower part of the slope, and the biota adapted to these conditions is 

 pushing up the slope. 



///. Mountain Top. On the bare mountain top, the forces of weather- 

 ing tend to break up the rock into a soil that is washed or blown away nearly 

 as fast as it is formed, except along the joint planes. 



The flora consists of rock inhabiting lichens and a number of crevice forms, 

 the fauna of the types frequenting open habitats, such as the grasshoppers, 

 etc. 



The soil formed in the crevices is in part held by plant roots, and, since 

 these soil areas are the habitats of the crevice forms of life, as they gradually 

 spread out, the habitats of the rock inhabitating forms are restricted. At 

 the same time, the soil washed down the slope is held at the forest margin, 

 thus permitting the forest societies to encroach on the crevice forms which 

 will thus in time tend to become replaced by the forest forms. This succes- 

 sion of forms is modified by the influence of the strong winds which tend 

 to break down the pioneer trees on the exposed areas. 



///. Cliff and Talus Slope. The cliff is constantly exposed to the forces 

 of weathering, for as fast as the rock is broken up it is removed, exposing 

 a fresh surface. 



The fauna and flora, owing to the unstable and exposed conditions, are 

 very, poor except on the rock ledges, where a slight soil accumulates. 



The talus slope at the foot of the cliff is also very unstable and is exposed 

 to the destructive effects of falling rock fragments. Toward the bottom, 

 beyond the limit of falling rocks, the blocks tend to become broken up into 

 a soil. 



