SEDENTAEY MATEEIALS: BESIDUAEY DEPOSITS 295 



prevailing type of the residuary deposits throughoiit widely 

 separated areas. It will be noted that silica as a rule exceeds 

 all other constituents, while alumina, iron oxides, and moisture 

 make up the main bulk of the residue. This generalization 

 holds good for nearly all sedentary soils, whatever the character 

 of the rocks from which they were derived, and is the more 

 pronounced the more advanced the decomposition. 



Columns I, II, III, and lY of this table (see opposite page) 

 are limestone residuals from southern Wisconsin. Columns I 

 and II are from the same vertical section, I being 4^ feet from 

 the surface, and II 8^, and in contact with the underlying lime- 

 stone. Columns III and lY are similarly related, III being 3 

 feet from the surface, and lY 4^ feet, the lower sample lying on 

 the unchanged rock. The larger percentages of silica in the 

 samples from nearest the surface indicate a higher state of 

 decomposition, the soluble constituents having been more com- 

 pletely removed. The presence of large percentages of alkalies 

 in these same samples indicates that these salts existed in the form 

 of silicates which have resisted the decomposing influences, and 

 remain mechanically included in the residues. Column Y is a 

 clay from the decomposition of the Knox dolomite at Morris- 

 ville, Alabama ; YI the characteristic red earth from the decom- 

 position of coralline limestone on the islands of Bermuda; YII 

 a product of the decay of a diabase dike at Wadesboro, North 

 Carolina; YIII a gabbro sub-soil from Maryland; IX a sub-soil 

 from the decomposition of Trenton limestone near Hagerstown, 

 Maryland; and X a residual soil from the decomposition of a 

 Triassic sandstone, Maryland. 



A microscopic examination of the material represented by 

 analyses I and lY, as given by the authorities quoted, showed 

 it to consist of particles in an extreme condition of comminution. 

 An actual measurement of over 700,000 of these particles yielded 

 results as below : 



Particles less than .0025 mm. in diameter 721866 



Particles between .0025 mm. and .005 mm. in diameter .... 9812 

 Particles over .005 mm. in diameter 0634 



732312 



Of those over .005 millimetre in diameter, particles reaching 

 0.06 millimetre were not rare. Nearly all those above 0.1 milli- 

 metre were found to be of flints and cherts which graded up 



