CLAYS, THEIR ORIGIN, COMPOSITION AND VARIETIES. 47 



globe is quartz or crystallized silica. Quartz, including the silica derived 

 from it, forms one quarter of the portion of the earth which we know as 

 its crust. Next to quartz in abundance is felspar, or rather the group ot 

 felspathic minerals. There are three or more well-marked divisions of 

 the felspar family, viz., orthoclase or potash felspar, albite or soda felspar, 

 and anorthite or lime felspar. All of these are complex minerals. 

 Orthoclase may be taken as the type of the group. Its composition is 

 thus expressed in chemical terms, K 2 ,0, A1 2 3 6 Si0 2 . In other 

 words it is a double silicate of potash and alumina. It appears that in 

 nature, when the opportunity for original composition among the 

 elements was offered, their natural affinities were never satisfied with 

 the formation of the simple compounds of silica and alumina, or silica 

 and potash, but these substances were obliged to enter into double 

 combination. As above described, the case is really much more complex 

 than has been here represented. Silicates of both potash and soda and 

 also of lime and magnesia are all found in the same mineral aggregation, 

 which must be designated by one or the other of the general terms used 

 in the classification of the felspars. But one of the substances named 

 above is quite likely to be in the ascendant and gives character to the 

 compound. But if the silicate of alumina is to be sought for in a com- 

 plex mineral, how does it attain the separate existence in which it 

 becomes so serviceable to us? In other words, how does clay originate 

 from felspar? 



Clay is a product of the decomposition of felspar through the agency 

 of the atmosphere. Though seeming so bland and harmless, the atmos- 

 phere is charged with agencies that will dissolve the firmest rocks of the 

 earth's crust. The oxygen, which is one of its main constituents, the 

 carbonic acid and the water that are always present in it, though the latter 

 occur in small and in varying proportions, constitute, when taken 

 together, an almost universal solvent. Soil waters also, containing the 

 acid products of vegetable decomposition, become a powerful agent of 

 rock disintegration and decay. Under the combined agencies above 

 noted granite rocks, and particularly those containing potash fel- 

 spar, pass through a rapid process of decomposition. At least one phase 

 of the process is the decomposition of the silicate of potash of the com- 

 pound mineral, through the agency of the carbonic acid of the air. The 

 carbonate of potash thus formed is soluble and is removed as fast as set 

 free in drainage or in surface waters, while the silicate of alumina 

 remains behind as clay. The granite rocks of which the felspar was a 

 leading component contain also quartz and mica and various other min- 

 erals, some of them in relatively large amounts. These accessory min- 

 erals become blended with the silicate of alumina in the process of 

 decomposition; and it is the product of this result that we call clay. 

 The silicate of alumina is its base and its characteristic element, but dis- 

 seminated in the mass are grains of quartz, flakes of mica and indefinite 



