Mineral Plant Foods contained in Soils. 353 



It will be noted that the phosphorus percentage in this 

 glacial soil is of very nearly the same value as that shown 

 by the averaged Maryland residuals ; while the potassium 

 ranges about the figure computed for the average rock of the 

 lithosphere. It would seem from this that the phosphorus 

 in the glacial material was either originally present in lesser 

 amounts (the rocks from which the Illinois glacial soils are 

 derived were largely shales and sandstones), or else the phos- 

 phorus was leached much more rapidly than the potassium. A 

 comparison of the Maryland residual soil averages and those of 

 the youngest Illinoian glaciation, on the basis that the former 

 represents average rock material chemically decomposed, while 

 the latter typifies such material derived mechanically, fails of 

 exactness in that some of the Maryland residuals are very prob- 

 ably derived from rock crumbling on steep slopes, i.e., mechan- 

 ical weathering and erosion, without complete decomposition ; 

 and in that the glacial soils were laid down unconsolidated, 

 and were, therefore, very permeable from the first to water, 

 and susceptible to rapid leaching. For these reasons a com- 

 parison of the youngest glacial soil in Illinois with that from 

 the oldest glaciation will express much more exactly the effect 

 of continued leaching on soils, for in this case the original 

 material can be considered identical. In all probability the 

 direction of the glacial advance in the several glaciations cited 

 was uniformly from the Labrador center of dispersion. It may 

 be assumed, therefore, that the glacially transported material of 

 the several glaciations was derived from essentially the same 

 bed rock. It has been found, also, that in the peripheral 

 deposits of the continental glaciations, a large percentage (87 

 per cent in one case) of the whole are of local origin, that is, 

 composed of material derived from bed rock underlying and 

 near -adjacent to the area surveyed.* Probably the finer par- 

 ticles of sand and rock flour have a more distant source, and 

 therefore represent a greater commingling of material from 

 widely different rock types. As to original constituents these 

 soils should, accordingly, be essentially similar both in the deri- 

 vation of their coarse and fine particles. The figures given 

 below, bulk analyses of soils from undulating prairie lands, 

 consisting of "gray silt loam on tight clay" deposited by the 

 earliest glaciation, the Lower Illinoian, may, therefore, with 

 confidence as to the essential similarity of original constituents, 

 be compared with the figures given above for the brown silt 

 loams of the Late Wisconsin glaciation. 



*Alden W. C, The Delavan Lobe of the Lake Michigan Glacier of the 

 Wisconsin Stage of Glaciation and Associated Phenomena. Prof. Paper 34, 

 TJ. S. Geological Survey, 1904, pp. 86-67. 



