PE0PEKTIE8 OF CLAYS 137 



General remarks on clay 



Clay is a hydrated silicate of alumina, having the formula 

 AI3 O3, 2Si03 + 2H3O, or silica 46.3 per cent., alumina 39.8 per 

 cent., water 13.9 per cent. This is the mineral kaolinite, and it 

 generally results from the decomposition of granite or other 

 feldspathic rocks. The three essential component minerals of 

 granite are quartz, feldspar and mica, and of the feldspars, 

 orthoclase or the potash feldspar is most commonly present. 

 The surface waters in percolating through the rocks attack the 

 feldspar and leach out the potash as carbonate, or possibly as a 

 silicate if the amount of carbonic acid is small. This breaking 

 up of the feldspar destroys the bond for the quartz and mica and 

 the rock begins to crumble. As a further result of the leaching 

 some silica is set free and left in a hydrated condition. The 

 alumina of the feldspar, the hydrated silica, and some water unite 

 forming kaolinite, the basis of all clays. A deposit of this kind 

 formed in place is a residual clay and its purity is largely 

 influenced by the composition of the parent rock and mechanical 

 conditions, both of which vary. Usually many accessory 

 minerals are present, and destroy the purity of the clay or kaolin. 

 In the general erosion of the land the kaolin together with other 

 minerals composing the rocks are washed into the lakes or ocean, 

 the coarser material transported by the streams being dropped at 

 their mouths to form deltas, while the fine aluminous mud settles 

 in the quieter waters forming a bed of clay. Such deposits of 

 clay are called " sedimentary " to distinguish them from the 

 " residual " ones. 



Beds of clay of vast extent are thus deposited in the ocean. 

 With further deposition they become buried far beneath the 

 ocean floor, where subjected to the action of dynamic agencies 

 such as heat and pressure these beds of clay become converted to 

 shale. With subsequent elevation of the sea bottom, and erosion 

 of the new land surface the shale becomes exposed as we now 

 find it over a large portion of the state. The disintegrated out- 

 crops of shale have often been used for brickmaking, having 

 been mistaken for deposits of sedimentary clay. Much aluminous 

 rock flour was produced from the erosion of the shales by the 

 ]S"orth American ice sheet. This was held in suspension by the 

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