99. 



to weathering. They decompose gradually, a pure potash (K) feldspar 

 will weather in place and change to kaolin, which is more like chalk 

 than clny. 



Clay is a remarkable substance; there is nothing just like it. 

 Kaolin when pulverised vill, with the addition of water, form clay. 

 Clay in fresh water does not settle easily. Clay in a fresh water river 

 may strike the salt water of the ocean, and then it precipitates like 

 the curdling of rrdlk (f locculation) . It joijjs together in flakes or 

 crumb a. Two phases of f locculation may be expressed thus: 

 1) Clay s*4zes silt particles nnd incrustif them; 

 2} It joins two particles of clay together. 



iie above action of clay frequantly takes i lace in the Mississi- 

 ppi River in the south. 



Here also are rnud banks penetrated by steamer channels. They 

 really are clay banks with variable amounts of silt from the river. If 

 pressure be added they form shale; add water and he?>t and this produces 

 a metamorphosis to hard bare rock back to feldspar (?). (Quartz, mica, 

 silicates of ( Al) ,f eldspar.and back again, etc.) 



3) Hornblende and pyroxenes: hornblende represents the amphi- 

 boles, augite represents the pyroxenes. They are silicates of Mg with 

 also CaO, Pe, and Al. present. 



There ^ro t > o types of these minerals one being rich in Al, the 

 other being poor in Al . The black color is due to Pe and therefore they 

 ther readily with water and air, as the iron oxidizes. Thus the soil 

 recruit; it;-> If with minerals from the decomposition of rocks. 



Those minerals give color to soils. They are generally associa- 

 ted with quartz and feldsoar . 



4) I'icas: micas occur in granites, especially gneisses and 

 schist o, in which latter they are the most abundant. Mica and quartz 

 form most of the schists. Micas do very little for our soils; they 

 occur in connection with quartz, and therefore a poor soil may be due 



to * $&&& coarse quartzite. 



5) Zeolites: are silicates containing water (hydrous sili- 

 cr-tes), and are secondary forms. They may be spoken of as the original 

 rocks in process of decompositions. They occur everywhere with decom- 

 posing feldspars. Zeolites are very important because they prevent 

 leaching of salts in the soil; they readily remove bases with the sub- 

 stitution of other bases, and aince they are rather easily soluble in 

 strong mineral acids, the bases so combined are more readily available 

 to plants than in most combinations found in the soil, and yet are not 

 readily leeched out of it. 



6) Calcite (CaCO^): often limestones come from the shells of 

 animals, as snails and mussels. Diatoms build walls of silica which are 

 ordinal! ty Indissoluble. Nature works under tremendously long periods 

 of tire. 



''rater containing carbon dioxide (CO^) will dissolve shells and 

 thus limestones often do not show shells. Shells in river and ocean mud 

 are not pure but are mixed with silt, sand and clay; they arc ,.;ood for 

 soils. 



7) Dolomite (MgfCaJCO^): may contain as high as 4^ of lig. 



