4 CLAYS AND CLAY INDUSTRY. 



but others in all stages of decay, and representing chemically 

 many different compounds, such as oxides, carbonates, silicates, 

 hydroxides, etc. (See Chap. III.) 



The discussion of the chemical and physical properties of the 

 clay will be left until later, but it may simply be stated here that 

 two prominent minerals in clay are quartz (silica) and kaolinite 

 (a mixture of water, silica and alumina, and known chemically 

 as a hydrated silicate of alumina). 



ORIGIN OF CLAY. 1 



Clay results primarily from the decomposition of other rocks, 

 and very frequently from rocks containing feldspar, so that for 

 this reason most writers have stated that it was derived from 

 feldspathic rocks. There are some rock species, however, that 

 contain no feldspar (such as serpentine), and others with very 

 little (as some gabbros), which, on weathering, produce some of 

 the most plastic clays known. In all of these clays there is found 

 a variable amount of the mineral kaolinite, which is of secondary 

 origin, i. e. } it is derived from other minerals by decomposition. 

 This is termed the clay base. 



In order to trace the process of clay formation, let us take the 

 case of granite, a rock which is commonly composed of three 

 minerals, viz., quartz, feldspar, and mica. When such a mass 

 of rock is exposed to the weather, minute cracks are formed in 

 it, due to the rock expanding when heated by the sun and con- 

 tracting when cooled at night, or they may be joint planes 

 formed by the contraction of the rock as it cooled from a molten 

 condition. Into these cracks the rain water percolates and, when 

 it freezes in cold weather, it expands, thereby exerting a prying 

 action, which further opens the fissures, or may even wedge off 

 fragments of the rock. Plant roots force their way into these 

 cracks and, as they expand, in growth, supplement the action 

 of the frost, thus further aiding in the breaking up of the mass. 



1 In this chapter the different kinds of clay are discussed according to their 

 origin. A classification according to uses is given in the Introduction to 

 Part III. 



