RECOGNITION OF CHARACTER OF SOILS. 505 



poses. 1 The gray or white " crayfishy ' bottom and bench 

 lands of the Southwestern States, so poor in lime, phosphates 

 and humus as to be worthless under existing conditions, are 

 characterized by an easily recognized scrubby growth of Water 

 and Willow Oaks (Q. nigra. or aquatica, and phellos), with 

 low, rounded tops ; while the same trees, when well developed, 

 indicate highly productive lands. 



Physical vs. Chemical Causes of Vegetative Features. The 

 extent to which the modifications of form alluded to above are 

 referable to chemical and physical causes respectively, can be 

 approached by the discussion of the presence or absence of cer- 

 tain trees from soils of extreme physical character, but other- 

 wise normally constituted. As has been shown above, the 

 black-jack and post oaks belong, as species, equally to the 

 heaviest and lightest soils within the state of Mississippi; to 

 the black and yellow " prairie " soils, as well as to the sandy 

 ridges of the yellow-loam region ; showing for these two species 

 as such, an independence of physical conditions and an ex- 

 traordinary adaptability, found in few other trees. They are 

 frequently found either alone or associated with only a few 

 other species of local adaptation, such as, in the prairie lands,, 

 the crab-apple, wild plum, and the juniper or red cedar. On 

 the soils of intermediate or loam character, on the contrary,; 

 they are always associated with other oaks as well as with 

 hickory, and in that association attain what may be considered 

 their normal type or form. 



.From the fact that the dense, rounded top is formed by the 

 black-jack oak both on the rich prairie lands and on the poor 

 soils of the Flatwoods, it would seem that that form is the out- 

 come of a physical cause, viz, the extreme " heavy-clay " char- 

 acter of both kinds of land ; and we may note that exactly the 

 the reverse effect is observed in the form growing on the poor 

 sandy ridges, as shown in fig's 79 and 80. Yet it will also be 

 noted that in the case of the post oak, the poor, heavy-clay 

 soil of the Flatwoods produces an open, broom-shaped top, 

 while the form assumed on the sandy ridges is substantially the 

 same for both species. Care must therefore be exercised in 

 drawing general conclusions as to the effects produced by 

 either physical or chemical causes, alone, upon tree forms. 



1 Rep. of Geological Reconnoissance of Louisiana; New Orleans, 1873, P- 2 7- 



