CLAYS OF KEW YOKK 507 



As calcite effervesces wlien moistened witli muriatic acid, its preS'- 

 ence in the clay may often be detected by the addition of this 

 cliemical to it. Calcite may be scattered tbrongli the clay in 

 the form of small grains or be present as concretions (commonly 

 called " clay dogs"). It not infrequently happens that some lay- 

 ers of the clay contain a much larger percentage of carbonate 

 of lime than others, and indeed, with a very great increase in the 

 amount of carbonate of lime, the clay might pass into a marl. 

 Where a deposit of clay rests on a bed of limestone, the lower 

 layers of the material may be more calcareous than the upper • 

 ones. The carbonate of lime found in clays is at times derived 

 from particles of limestone if the clay is a sedimentary one, or in 

 the case of either sedimentary or residual clays it may come fro'm 

 the decomposition of lime soda feldspars, or again it may be in- 

 troduced by percolating waters. 



Gypsum may be present in the clay as grains, needles, or well 

 foraied crystals, or lamellar masses. It is much softer than calcite, 

 being scratched by the finger nail, often has a pearly luster, is 

 transparent, and does not effervesce with acid. In hard burned 

 bricks gypsum simply acts as a flux, but in lightly burned ones, it 

 gives rise to soluble sulfates, which cause efflorescence. In the 

 salina shales it often forms large transparent plates. 



Mica. This can frequently be easily detected by the naked eye, 

 even though it may be present in a very finely divided condition, for 

 the small scales of it have a high luster. Mica is seldoin absent 

 in clays and is usually present to a greater or less extent in the 

 best known kaolins. Owing to its nature it floats very easily, 

 and is consequently very hard to eliminate by washing. As white 

 mica is very refractory, and when finely ground possesses a certain 

 amount of plasticity, its presence in small amO'im.ts is not very 

 injurious. 



The mica found in clays is generally derived from igTieous or 

 metamorphic rocks, such as gTanites, gneisses, or schists. Two 

 kinds of mica ate commonly found in clay, namely biotite and 

 muscovite. The biotite mica is a silicate of iron, magnesia and 



