48 CLAYS AND CLAY INDUSTRY. 



mineral with a tendency to split into rhombohedral fragments, 

 due to the presence in it of several directions of splitting or 

 cleavage; it is also' soft enough to' be easily scratched with a 

 knife. Few clays contain grains of calcite sufficiently large to 

 be seen with the naked eye, although in some the calcite, as 

 well as some other minerals, may form concretions. 1 In some 

 swamps, clay beds are found which are highly charged with lime 

 carbonate, and known as marl (not to be confused with the 

 Greensand marls of the Cretaceous). Very little lime carbonate 

 is found in the New Jersey clays, except in those of glacial origin. 



Gypsum. — This mineral, the hydrous sulphate of lime, con- 

 tains lime (CaO, 32.6 per cent.), sulphuric acid (S0 3 , 46.5 per 

 cent.) and water (H 2 0, 20.9 per cent.). It may occur in clays, 

 even in large lumps, but, so far as known, these have not been 

 found in New Jersey. Gypsum, when present in clay, and large 

 enough to be visible without the use of a microscope, forms 

 crystals or plate-like masses. It is much softer than calcite and 

 can be scratched with the finger nail, has a pearly lustre, is trans- 

 parent, and does not effervesce with acid or vinegar. When 

 heated to a temperature of 250 C. (482 F.), the gypsum 

 loses its water of combination, and, when burned to a still higher 

 temperature, at least a part of the sulphuric acid passes off. 



Hornblende and Garnet. — These are both silicate minerals of 

 complex composition, which are probably abundant in many 

 impure clays, but their grains are rarely larger than microscopic 

 size. Both are easily fusible and weather readily, on account 

 of the iron oxide in them, and, therefore, impart a deep red 

 color to clays formed from rocks in which they are a prominent 

 constituent. 



Dolomite. — Dolomite, the double carbonate of lime and mag- 

 nesia, and also magnesite, the carbonate of magnesia, may both 

 occur in clay. They are soft minerals resembling calcite, and 

 either alone is highly refractory, but, when mixed with other 

 minerals, they exert a fluxing action, although not at so low a 

 temperature as lime. 



1 See Limonite, Siderite. 



