CLAYS OF NEW YOKE: 509 



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metallic scales, or as a superficial coating on other mineral grains. 

 It dissolves quietly in muriatic acid. Iron may also' occur in clay as 

 a constituent element of other minerals, and indeed the effect which 

 it produces is dependent not so much on the actual amount of iron 

 oxid present as on its condition, according as it is combined with 

 silica, carbonic acid or some other acid. 



Hornblende. This is not an unco^mmon constituent of clays, and 

 when present is generally in the form of tiny scales or flakes of a 

 dark green color, showing transparency under the microscope only 

 when extremely thin. It is highly probable that the hornblende 

 does not remain very long as such, for it deco^mposes quite easily, 

 yielding hydrated ferric oxid or limonite. 



Rutile is probably of widespread occurrence in clays, though 

 never in large quantity. It occurs mostly in the form of bristle- 

 like crystals. 'No systematic study of their occurrence in clay has 

 ever been taken up. The writer has observed them in some of 

 the Staten Island clays, and reference has been made to them from 

 time to time by other writers. {See J. J. H. Teall. Min. mag. 

 7: 201; G. E. Ladd. Amer. geol. Ap. 1899) 



Vanadiates, though not as co-mmon in clays, may cause disr- 

 coloration. In Germany they have been found in clays associated 

 with the lignites, and also in some fire clays, but in. this country, 

 so far as the writer is aware, they have never been investigated. 

 Clays containing soluble vanadiates, if not burned at a sufiiciently 

 high temperature, will show on the surface of the ware a green 

 discoloration, which, though it can be washed off with water, will 

 continue to return as long as any of the salt is left in the brick. 

 Vanadiates may be rendered insoluble by burning the clay to a 

 point of vitrification. (Soger's Ges. Schrift. p. 301) 



Other minerals may occur in clays, such as magnetite, titanite 

 etc., but the quantity is small. 



Organic remains. These consist of bituminous matter, roots, 

 amber and other substances, which volatilize on ignition. 



