SEDENTARY MATERIALS: RESIDUARY DEPOSITS 291 



and nocMar masses of eliert which, existed as meclianically ad- 

 mixed impurities. 



The inherited characteristics of deposits of this nature may 

 be illustrated by the accompanying exaggerated section across 

 central Kentucky where, it is easy to see, the regolithie mate- 

 rial overlying the Lower Silurian and Cambrian limestones may 

 contain a portion of all the insoluble residues from the hundreds 



EiG. 27. — ^Diagram showing the successive variations of fertility in tlie 

 soils of central Kentucky during the downward movement of the rocks. 

 a, a, a, parts of the present surface enriched by deca;f of limestones; h, 

 next preceding stage, when soils rested on Devonian shales and were moder- 

 ately fertile; c, yet earlier stage, when soils were formed on millstone 

 grit and were very lean; d, earliest stage when soils rested on the coal 

 measures, and were moderately fertile. For simplicity of illustration 

 several stages of variation are omitted. After N. S. Shaler. 



of feet of Upper Silurian, Devonian, Lower and Upper Carbon- 

 iferous beds which formerly stretched above them. Upon the 

 nature of this inheritance must depend the adaptability of the 

 regolith to soil purposes and its consequent fertility.^ 



The transition from a regolith of this type to fresh rock is 

 usually quite sharp, owing to the fact that limestones decompose 

 mainly through solution from the immediate surface. Never- 

 theless there is a gradual change in the character of such a 

 deposit from above downwards, owing to the oxidizing influence 

 of the air and percolating waters. (See p. 243.) 



As above noted, the mineral particles in the older residuary 

 deposits are, with the exception of the quartz, found to be as a 

 rule in a state of advanced decomposition. Nevertheless the 

 ultimate individual constituents of even the darkest clays of the 

 driftless regions of Wisconsin, as examined by Messrs. Chamber- 

 lin and Salisbury, are transparent, although stained by iron 

 oxides. 



Concerning the physical properties of limestone residues as 

 occurring in this driftless area, the following statements are 



^ The limestones of the Boone formation, near Talequah, Indian Territory, 

 contain so large an amount of chert nodules, as to render the residual soil 

 unfit for cultivation, and suitable only for the growth of forest trees, (J. A. 

 Tafet, Folio 22, U. S. Geol. Survey.) 



