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with the spheroidal structure already noticed ; 

 when they are separated by empty intervals, or 

 by intervals filled with another substance. 



The contact is sometimes such as to admit of 

 the ready separation of the prisms ; at other times, 

 there is a partial or more complete coalescence. 



The sizes of prisms are various. 



In diameter, or thickness, I have not found 

 examples exceeding 9 feet ; and, from that, they 

 vary to one foot, or less ; in columnar ironstone, 

 (which, as a modification of shale, is here ranked 

 with rocks) they are sometimes even less than the 

 tenth of an inch, so that the mass becomes nearly 

 fibrous. 



In length, they vary from one, even to 300 

 feet, or much more ; but when the length be- 

 comes so considerable, it is difficult, for want of 

 access, to determine truly whether the single 

 prisms of a mass are continuous throughout. 



The forms are equally various, from three 

 sided, upwards, even to twelve ; but figures of 

 four, five, and six sides, are the most common. 

 Such figures are by no means regular, unless in 



