THE MATERIALS OF STRATA. 43 



Under such circumstances, the chemical composi- 

 tion of a clay is sometimes the same as that of a 

 sandstone; and there maybe an unbroken tran- 

 sition from one deposit to the other through an 

 intervening loam. Almost all the minerals which 

 enter into the composition of crystalline rocks, 

 arc occasionally found in clays. Often the origin 

 of a clay is revealed in the abundance oi the 

 flakes of mica which are scattered through it, for 

 the silicate of alumina corresponds chemically 

 with decomposed felspar, and the presence of 

 particles of quartz diffused in it, shows that the 



rock has been derived from an old crystalline 

 material. The presence of mica renders it prob- 

 able that the schists which were denuded were of 

 the kind termed gneiss, if the crystalline rock 



was not granitic. Some ancient clays have been 

 found full of needles <>i the mineral rutile. The 

 purest clays are formed on land from the decolla- 

 tion of white granite. Such a source would 

 appear to be necessary for the beds of white pipe 

 clay, found interstratihed in the Bagshot sands of 

 I lampshire and I tors 



The colours of clay are due to oxides of iron. 



asionaHy, as in the Woolwich and Reading 



beds at Reading, current bedding is marked by 

 alternate layers of red and green clay. Such bed- 

 ding would be unsuspected, but for the colour. 

 Often a blue clay passes into a brown or yellow 

 tint. It is then sandy and porous, so as to per- 

 mit the infiltration of water charged with atmos- 

 pheric air, which oxidises the iron. 



Minor sources for clay are the small percent- 

 age of silicate of alumina which is the insoluble 

 residue left when limestones have been dissolved. 

 This forms the red cave earth in limestone dis- 



