61 



He had told us (page 222), that the prisms are always 

 vertical: and again, (page 231), he generalizes, and makes 

 this not an accidental position, but a matter of necessity; 

 assuring us, that, " in lava, the result must be a vertical 

 " prism." 



Had Mr. Desmarest paid any attention to basalt, he 

 would have known, there are different varieties of it, which 

 directly contradict his rule, so dogmatically laid down: 

 some varieties, in which the axes of the prisms arc in- 

 different to all positions; while the stratum, in which they 

 are placed, remains perfectly steady; the axes assuming 

 different degrees of obliquity, at short distances, and some- 

 times undulating. 



The prisms, too, of the basalt walls, commonly called 

 wJiynn dykes, are ahvays horizontal. 



VOL. X. 



PART 



