40 



popularly reputed to form the very best soils. " A limestone country 

 is a rich country " is a popular axiom to which there are, on the whole, 

 but few exceptions. 



Origin. Actual observation of what is happening at the 

 present time, as well as the examination of the rock as anciently 

 formed, prove conclusively that with insignificant exceptions, 

 all limestones have been formed from the framework and 

 shells, and to some extent from the bones, of marine and 

 fresh-water organisms, ranging in size from the extinct giants 

 of the lizard relationship to those recognizable only by the 

 microscope. Owing to the solubility of lime carbonate in car- 

 bonated water, the organic forms have often (in crystalline 

 limestones) been almost completely obliterated in some por- 

 tions, but in others are so preserved as to prove undeniably 

 the similarity of origin of the whole, and that they have been 

 formed in relatively shallow water, as they are to-day. 



Impure Limestones as Soil-formers. From what has been 

 said regarding the composition of sea-water, it will readily be 

 inferred that a pure deposit of any one kind cannot easily be 

 formed in it; moreover, the matter held in mechanical sus- 

 pension everywhere near the coasts must very commonly be 

 included within the calcareous deposits formed off-shore. 

 Hence few limestones dissolve in acids without leaving a 

 residue of sand, clay and various other substances, usually 

 even some organic matter not fully decomposed; sometimes 

 less than half of the mass is really lime carbonate. It is 

 obvious that when the solvent action of carbonated water is 

 exerted upon such impure limestones, a loose residue of earthy 

 matters will remain behind. It is by this process that a con- 

 siderable proportion of the richest soils in the world have been 

 formed, which have given rise to the popular maxim above 

 quoted. They are emphatically " residual " soils ; sometimes, 

 it is true, somewhat removed, by washing-away, from their 

 point of origin, but in many cases forming a compact soil- 

 layer on top of the unchanged rock, into which there exists 

 every shade of transition. Striking examples of such residual 

 soils in place are seen in the black prairies of the southwestern 

 United States; they are mostly rather "stiff" (clayey), and 

 hence has arisen a local popular error, to the effect that clay 



