THE VARIOUS ROCKS AS SOIL-FORMERS. 53 



The soils derived from trachytes and trachytic lavas are 

 generally light-colored and light in texture; the latter from 

 the presence of a large proportion of volcanic glass, together 

 with undecomposed crystalline minerals. These are usually 

 rich in potash, but poor in lime and phosphates. The high 

 quality of the wines of the lower Rhine has been ascribed to 

 these soils, which however vary greatly within the areal limits 

 of the production of the high-grade wines, not only from gray 

 trachytes to dark colored, highly augitic basalt, but also to 

 acidic quartz porphyries or rhyolites, and clay-slates. 



The rhyolites on the whole yield the poorest soils among the 

 eruptive rocks; they are slow to weather at best, and the soils 

 produced are poor and unsubstantial, largely from the predom- 

 inance of quartz and undecomposable, glassy material ; of which 

 the phonolites are the extreme type, resisting the influence of 

 the atmospheric agencies just as would so much artificial glass. 

 Soils consisting largely of volcanic glass may be found cover- 

 ing considerable areas in the Sierra Nevada of California. 

 Such " volcanic ash " soils are usually very unthrifty, and bear 

 a growth of small pines. 



Soils from sedimentary rocks. Limestones, when pure and 

 hard, are very slow to disintegrate, and are also very slowly 

 attacked by carbonated water (see chap. 3. page 41). Soft 

 impure and vesicular limestones are, however, very rapidly at- 

 tacked, especially when underlying a surface clothed with the 

 luxuriant vegetation that usually flourishes on soils rich in 

 lime. The popular adage that " a limestone country is a rich 

 country," is of almost universal application and stamps lime, 

 from the purely practical standpoint, as one of the most im- 

 portant soil ingredients. 



Residual Limestone Soils. Striking examples of the forma- 

 tion of large, fertile soil areas by the leaching out of limestones 

 are found in the States of Alabama, Mississippi, Louisiana and 

 Texas, where the fertile black prairies have been largely thus 

 formed. The '' blue-grass " country of Central Kentucky is 

 another case in point. 



The following table shows a representative example of the 

 relative composition of the (cretaceous) '' Rotten Limestone'' 

 of Mississippi, and the " residual " soil-stratum derived from 

 it. The average thickness of the layer of residual clay above 



