CHAPTER XXVII. 

 CLAYS, SHALES, AND SLATES. 



EXCEPT when a very clayey limestone or a slag is one component of 

 a Portland-cement mixture, the silica, alumina and iron oxide necessary 

 for the mix are always supplied in the form of clay, shale, or slate. 



The materials known respectively as clays, shales, and slates are 

 of practically the same composition and ultimate origin, but differ in 

 their degree of consolidation. 



Clays are ultimately derived from the decay of older rocks, the 

 finer particles resulting from this decay being carried off and deposited 

 by streams along their channels, in lakes, or along parts of the seacoast 

 or sea-bottom as beds of clay. In chemical composition the clays are 

 composed essentially of silica and alumina, though iron oxide is almost 

 invariably present in more or less amount, while lime, magnesia, alka- 

 lies, and sulphur are of frequent occurrence, though usually only in 

 small percentages. 



Shales are clays which have become hardened by pressure. The 

 so-called "fire-clays" of the Coal Measures are shales, as are many 

 of the other "clays" of commerce. The slates include those clayey 

 rocks which through pressure have gained the property of splitting 

 readily into thin parallel leaves. 



Clays. The term clay is applied to fine-grained unconsolidated 

 materials which possess the property of plasticity when wet, while they 

 lose this property and harden on being strongly heated. Being, as 

 explained below, the finer debris resulting from the decay of many 

 different kinds of rocks, the clays will naturally differ greatly among 

 themselves in composition, etc. 



Origin of clays. When rocks of any kind are exposed to atmospheric 

 action, more or less rapid disintegration sets in. This is due partly to 

 chemical and partly to physical causes. It is hastened, for example, 

 by the dissolving out of any soluble minerals that may occur in the 

 rock, by the expansion and contraction due to freezing, and by the 

 action of the organic acids set free by decaying vegetable matter. The 



353 



