PRINCIPLES OF CLASSIFICATION. 21 



In a featureless plain it might be difficult to decide 

 where to draw the line between different classes of soils, 

 but topographic diversity facilitates matters considera- 

 bly by affording distinctions between wet and dry soils, 

 ridge and valley soils, residual and colluvial soils, etc. 



Although the geological age of a rock may have little 

 direct influence on the vegetation growing above it, soil 

 and topography are so intimately connected with geology 

 that a geological map is of fundamental importance to 

 the student of forest geography, in some parts of the 

 world at least.* 



The correlations between geology, topography, soils 

 and vegetation, although not very evident in some 

 of the colder and hotter parts of the world, are 

 perhaps nowhere more clearly exhibited than in Ala- 

 bama; and the map accompanying this report does not 

 differ conspicuously from a geological map of the same 

 size. 



In the northeastern quarter of the state the chief dif- 

 ferences are due to the fact that in some of the valleys 

 several different Paleozoic formations crop out in a suc- 

 cession of long narrow belts which cannot be shown on 

 such a small map as this. And furthermore, while it is 

 possible to map the outcrops of formations where they 

 are only a few yards wide — if a sufficiently large scale 

 be used — , the soils are often less diversified than the un- 

 derlying rocks, or mixed (especially on steep slopes), 

 topography cannot be studied to advantage in an area of 

 less than several hundred acres, and it has not been 

 found practicable to have geographical divisions, of the 

 rank here considered, less than a few miles wide, or too 

 discontinuous, either. In the valley regions therefore 

 the various geological formations, though often very dis- 

 tinct, are regarded as indicating local forest types rather 

 than distinct regions. 



In the northwestern quarter there is a rather wide 

 transition zone between the coal region and the central 

 pine belt, where the unconsolidated strata of the latter 



*The diagram in Appendix A will make the relations of all these 

 environmental factors to each other and to the forests a little 

 plainer to students and other interested persons. 



