348 THE BEGOLITH 



common red earth of the islands. Though usually fertile, where 

 the leaching has been excessive the resultant soil is so rich in 

 iron and other deleterious constituents as to be quite barren. 



There are few more impressive facts in agricultural geology, 

 than that each foot in depth of such soil, as it now lies at our 

 feet, may indicate the removal of at least 100 feet in actual thick- 

 ness of limestone. In other words, even assuming that nothing 

 has been lost by mechanical erosion, the surface of the ground 

 has been lowered this much in bringing about the present con- 

 ditions. 



From what has gone before, it is obvious that soils derived 

 by purely mechanical agencies will, if unmixed with other ma- 

 terials, show a composition closely resembling the mother rock,, 

 as in the case of that derived from granite as described on p. 186 

 or those derived from argillites and siliceous sandstones ; others 

 in which chemical agencies prevailed may by solution and other 

 changes have so far lost important constituents as to be scarce 

 recognizable as rock derivatives at all. Obviously a rock mass 

 containing in itself none of the elements of plant food cannot, 

 merely through its decay, furnish soil of appreciable fertility. 

 This fact is well illustrated in the region known as the Bare 

 Hills north of Baltimore, Maryland, or the Chester County 

 Barrens in southern Pennsylvania. Both regions are under- 

 laid by peridotites — rocks rich in iron-magnesian silicates, but 

 almost wholly lacking in lime, potash, or other desirable con- 

 stituents. Such rocks not merely decompose very slowly, but 

 the stingy product of their decomposition consists only of hya- 

 line forms of silica, magnesian carbonates, or silicates and fer- 

 ruginous products quite devoid of nutrient matter, affording 

 food and foothold to scanty growths of grass and stunted 

 shrubs. That, however, a rock contains all the desired mate- 

 rials, is no certain indication as to character of its decomposition 

 product, since in the process of decomposition much desirable 

 matter may have become lost. Nevertheless most soils retain 

 what we may call inherited characteristics, and a direct com- 

 parison whenever possible is by no means uninteresting, as will 

 be noted later. 



It need scarcely be remarked that the value of any soil de- 

 pends wholly upon its capacity for plant growth. Hence a 

 satisfactory treatise on the subject should be written with a 

 view to showing to what this capacity is due, and what are 



