135 



a few accidental cases; and every modification 

 of form may be found aggregated in the same 

 mass. Nor are the sides of the prisms neces- 

 sarily straight ; being .sometimes convex, or 

 concave. 



Prisms are sometimes continuous for a con- 

 siderable length. At others, they are divided 

 by oblique or irregular joints. In many in- 

 stances, the joints are at right angles to the 

 axes, and occur at different intervals, from an 

 inch to many yards. When such joints possess 

 an average general distance varying from one to 

 three feet, a considerable appearance of artificial 

 regularity follows; and it must be remarked, 

 that the most perfect and numerous joints occur 

 in the most regularly formed columns. They 

 are in some cases, as already noticed, so frequent 

 as to produce tabular prisms, not reaching to, 

 or exceeding, an inch. 



In the joints, the surfaces in contact are 

 sometimes uneven, at others flat, at others again, 

 alternately concave and convex ; and either of 

 these forms may be found in the lower portion. 



