COMPOSITION OF MUDS. 889 



exposed to the weathering processes under more favorable circumstances 

 than any other mechanical deposit. Consequently mud is material in 

 which decomposition is farther advanced than in any of the previously 

 considered classes of mechanical sediments. It follows from this that 

 muds are likely to be deficient in the more readily soluble compounds. 

 Of these the alkalies stand first, and of the alkalies sodium is more largely 

 dissolved, since a large proportion of sodium in the original igneous rocks 

 occurs in minerals which are more readily decomposed than the minerals 

 which bear potassium — that is, sodium occurs largely in the nephelites, 

 sodalites, and basic feldspars, which, as shown on pages 252, 260, 292, 

 295-299, are readily soluble; whereas the great sources of potassium are 

 orthoclase and microcline, difficultly decomposable minerals. The mate- 

 rials are also apt to be depleted in calcium and magnesium, since the 

 alkaline earths are so readily soluble. (See analyses A, B, C, of shales in 

 Chapter XI, under " Composite analyses of sedimentary rocks." The 

 depletion in calcium usually has gone farther than the depletion in mag- 

 nesium, since in the belt of weathering much of the magnesia is retained 

 in the serpentines and talcs. The material may or may not be depleted 

 in iron. While aluminum and silica also have been dissolved in the belt 

 of weathering, the solution of these substances is less rapid than of the 

 others, and thus there is usually an increase in the relative amounts of 

 these elements. The relative rate of solution of the elements is more fully 

 considered on pages 507-518. 



The processes which lead to the solution are those of carbonation, 

 hydration, and oxidation, so frequently mentioned. From the muds the 

 carbonates are mainly removed in solution, the hydroxides are removed 

 to some extent, and the highly oxidized iron largely remains. All the 

 foregoing facts clearly appear when the analyses of original igneous rocks 

 are compared with those of muds. I reproduce from Clarke the first 

 column of a table giving the average of analyses of igneous and crystalline 

 rocks, made up from 830 analyses from various parts of the world, which 

 may be regarded as closely approximating the composition of the original 

 material from which the muds must have been derived, except that a con- 

 siderable number of the 830 analyses are from metamorphic rocks, a portion 

 of which may be metamorphic sedimentary rocks. So far as there was 



