IO 



CLAYS AND CLAY INDUSTRY. 



land. Occasionally a bed of clay may be extensively worn away 

 or corraded by currents subsequent to its deposition, leaving its 

 upper surface very uneven, and on this an entirely different kind 

 of material may be deposited, covering the earlier bed, and filling 

 the depressions in its surface. If the erosion has been deep, 

 adjoining pits dug at the same level may find clay in one case 

 and sand in the other (Fig. 3). 



While in many instances the changes in the deposit are clearly 

 visible to the naked eye, variations may also occur, due to the 

 same cause, which would only show on burning. Thus, for 

 example, the so-called retort clay, found in the Woodbridge 

 region, is similar in its plastic qualities wherever found, but the 



Fig. 3. 



Section showing uneven boundary between two clay beds, due to erosion of one before 

 the deposition of the other. 



shrinkage of that found in the different pits is not always the 

 same, because it varies in fineness from place to place. It may 

 also vary in color. Similarly, the Woodbridge fire-clay member 

 of the Raritan (Chapter VIII) contains a layer known as the top- 

 sandy in many pits of the Woodbridge district, but around Perth 

 Amboy it is wanting, either because it was not deposited there, 

 or, if it was, subsequent scouring action by ocean currents in 

 that portion of the area may have worn it away. In strong con- 

 trast to this we may take the black, micaceous, laminated sands 

 and clays, overlying the refractory members of the Woodbridge 

 fire-clay bed. These are found at Woodbridge, Perth Amboy, 

 South River and intermediate points, and were widely developed, 

 although, even here, the individual beds are very changeable. 



