20 Trnnesske Flora. 



The succession of strata is normal throucfhout: Uppermost sub- 

 carboniferous limestone and chert, followed by the Devonian shale; 

 lastly, the lower Silurian. 



Irregular basins, crossed and intersected by ridges of from 400 to 

 600 feet elevation, and this lower terrace again girded by a plateau, 

 is the outline of Middle Tennessee. This shape of surface is the 

 effect of unequal erosion through differently constituted strata. 

 This agency has been in bygone epochs, probably during the Cham- 

 plain, much more energetically at work than at the present day. 

 Some superficial gravel beds and the iron ores in the western part 

 of Middle Tennessee have probably been deposited at this period. 

 The floor of this denudation lies either in the aSTashville (Hudson) 

 or Trenton limestone, while the hilltops are Devonian or subcarbon- 

 iferous shales or chert, sometimes sandstones. The limestones pro- 

 duce the strongly calcareous, very productive soil of the lower 

 grounds. The disintegration of the Devonian shales resulted into 

 strata of heavy, impermeable beds of clay or loams, and the con- 

 comitant swampy lands and the cherty and siliceous beds have 

 yielded the angular gravels of the poor hilltops. The difference 

 of elevation is so slight that it cannot essentially affect vegetation, 

 and the greater or less adaptation only of plants to certain soils 

 causes their appearance or disappearance at the limits of particular 

 geological areas. The phosphatic rocks belong to this group. 



Alluvium is restricted to river and creek bottoms. The heavy 

 and fertile clay soils of the uplands are the insoluble residuum of 

 the fossiliferous, argillaceous limestones, with more or less com- 

 plete lixiviation of the lime by atmospheric precipitations. In the 

 midst of these is a third class of soil, of black color, full of bog iron 

 ore in the shape of rounded grains. Sulphurated ferrugineous 

 springs, decomposition of pyritical limestones, accompanied by 

 perennial growth of cane, have, as it seems, generated it. 



Increase in annual range of temperature and greater dryness 

 of air, as compared with the former regions, cause the mountain 

 flora to disappear and to yield to other designs in nature's garb. 

 A close botanical inquiry into the array of species soon discloses 

 the fact that different assemblies of species congregate in the lime- 

 stone and argillaceo-siliceous region. The former includes the 

 glades ; the latter, the barrens, of Middle Tennessee. 



Glades are thinly-wooded, unarable lands, with shallow soils, 

 fit only for pastures. They ought to remain in their natural state. 



