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In some remarkable examples of a lower concave 

 surface, the angles of the inferior portion pro- 

 trude in a point which covers a corresponding 

 deficiency in the upper. In others, equally re- 

 markable, the surface of a joint is marked by a 

 channel parallel to its boundary and near the 

 edge of the prism. The first of these is exem- 

 plified in the columnar traps, or basalts, the last 

 in columnar ferriferous shale, or ironstone. 



In the smaller columnar structure, the prisms 

 are sometimes longitudinally striated ; and, in 

 some instances, further distinguished by pro- 

 tuberant rings, or inseparable joints. Examples 

 of this may be found in limestone, in jasper, 

 and in argillaceous ironstone. 



In the act of decomposition, the portions of 

 jointed prisms sometimes give indications of a 

 lamellar structure, which, in the progress of 

 desquafnation, leaves a spheroidal nucleus ; each 

 successive lamina becoming gradually more re- 

 gularly curved. This case is analogous to that 

 which occurs in the common prismatic struc- 

 ture, already noticed, and admits of the same 

 doubts as to its real nature. 



