88 PECULIAR STRUCTURE OF CLARK'S CLAY-BEDS. 



Owing to this historic origin, the clay-beds should 

 rest upon the true glacial drift-deposits. Prof. Mather 

 has stated that this is everywhere the fact ; and the same 

 eminent geologist attributes the origin of the material 

 of the clay chiefly to the decomposition of the great slate 

 formations on the Hudson River. This is an essentially 

 different origin from that of Cretaceous clays of New- 

 Jersey, which Prof. Cook, the state geologist, attributes 

 to the decomposition of feldspars in Azoic granite rocks. 

 These differences in origin are sufficient to account for 

 the great diversity in their respective economic values. 



The thickness of this clay deposit is not generally over 

 one hundred feet, and frequently much less. But Prof. 

 Mather has stated that it is often two hundred feet deep 

 on tlie Hudson River. Along this river it is found in 

 two different layers, the blue and the gray. The blue 

 clay is always the lower and much the thicker of the 

 two. They are both generally calcareous, and, except 

 for local disturbance, horizontally stratified. 



The usual nearly horizontal deposition is well shown 

 at the extensive clay beds at the Dutchess and Columbia 

 Junction, which are of blue clay and at least two miles 

 in length along the bank, and rising about sixty-five 

 feet above the river level. The material here is quite 

 free from sandy layers and very homogeneous. Through- 

 out its whole thickness there is a curious and perfectly 

 regular alternation between layers of clay about five or 

 six inches thick and bands of layers of quite similar 

 material, of which each layer is about one quarter of an 

 inch thick. This clay is very adhesive and splits up 

 with perfect ease into quite thin and flexible slices, thus 

 presenting all the appearance of incipient shale which 

 has not quite reached the point of induration. It might 

 easily be mistaken for beds of shale which have become 

 softened by the elements. 



Sand and gravel-beds were also prominent elements in 



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