628 NEW YORK STATE MUSEUM 



CLAY-WORKING 

 Structure of clay deposits 



Residual clays. The mode of origin of these has already been 

 mentioned. Such a clay may occur either in the form of a broad 

 mantle over bed rock, , of variable depth and lateral extent, or it 

 may occupy the position of a vein cutting across the strike of the 

 other rocks or sometimes parallel with their bedding or lamination. 

 Residual clays of the first type are abundant in the upland regions 

 of the southern states and form the most abundant brickmaking 

 material of that part of the country. 



Residual deposits of the second type result commonly from the 

 decomposition of veins of granite or feldspar. They vary in width 

 from a few inches to several hundred feet. Their vertical extent 

 depends in most cases on the depth to which the weathering has 

 reached, except in the case of those kaolin deposits which have re- 

 sulted from action of subterranean vapors. (See " Origin of clay," 

 p. 496) Vein formations of kaolin seldom show great length, and 

 usually pinch out in both directions. In some localities they are 

 however known to be as much as 1000 feet long. They are com- 

 monly separated from the country rock by more or less sharp 

 boundaries, which are preserved even though the wall rock also be 

 decomposed, as it usually is. They frequently branch, and at times 

 contain lenses of quartz, which resist the weathering agencies and 

 stand out in bold relief on the surface. It rarely pays to work a 

 vein under 6 feet in width. 



Sedimentary clays. These occur in the form of beds either close 

 to the surface or interstratified with other deposits which have been 

 formed by water, such as sandstones. Deposits of sedimentary 

 clay do not pass gradually into the underlying rock as residual 

 clays do. In many parts of the United States sedimentary clays 

 form lens-shaped masses which are surrounded on all sides by sand. 

 The clay beds of Staten Island well illustrate this point, and the 

 conditions observed are caused by variations in the velocity of the 



