THE MAJOR SOIL-FORMING MINERALS. 33 



Formation of Clays. When instead of remaining in place, 

 this kaolin is washed away and triturated in the transportation 

 by water, it is partially changed from its original chalky con- 

 dition to that plastic and adhesive form which is the character- 

 istic ingredient of all clays. The remarkable properties of this 

 substance and the part it plays in the physical constitution of 

 soils, will be discussed in another chapter. Its lightness and 

 extreme fineness of grain (if grain it can be called) cause it to 

 be carried farther on by the streams than any other portion of 

 the products of rock-decomposition save those actually in 

 solution ; it can therefore be deposited only in water that is 

 almost or quite still (as in swamps) so long as the latter is 

 fresh. So soon however as brackish or salt water is en- 

 countered, clay promptly gathers into floccules ("floccu- 

 lates"), and thus enveloping the finest-grained silts that may 

 have been carried along with it, it quickly settles down, form- 

 ing the " mud banks " and heavy clay soils that are so char- 

 acteristic of the lower deltas of rivers, as well as of swamps 

 formed by the backwater or overflow of the same. 



When instead of potash feldspar alone, the lime- or soda- 

 lime feldspars are also concerned in the -decomposition process, 

 the resulting clay soils will be more or less calcareous, while 

 the soda, as stated above, is for the greater part leached out 

 permanently. 



Hornblende (Amphibole) and Pyroxene (Augite). These 

 are two very widely diffused minerals, differing but little in 

 composition though somewhat differently crystallized, mostly 

 in short columnar forms. The typical and most abundant 

 varieties of these minerals appear black to the eye, though in 

 thin sections they are bottle-green; they form the black in- 

 gredient of most rocks. 



The color is due to ferroso-ferric (magnetic) oxid of iron ; the mineral 

 as a whole may be considered as a silicate of lime, magnesia, alumina 

 and iron, varying greatly in their absolute proportions; alumina and 

 iron being sometimes almost absent. When iron is lacking the mineral 

 may be almost white (tremolite, asbestos), and its weathering is then 

 much retarded, since the oxygen of the air cannot take part in the pro- 

 cess of disintegration. 



The black variety of hornblende is not only the most abun- 

 3 



