105 



stratum, all of nearly the same degree of perfection; bnt, 

 when we compare diflerent strata, of the same variety, 

 the perfection or neatness of the work varies, until it passes 

 into an amorphous mass. 



Nature seems to have provided, as carefull}^ for the pre- 

 seiTation of the distinctive characters, of the different va- 

 rieties of basalt,, as she has done, to prevent confusion 

 in the several tribes of the animal and vegetable king- 

 doms. We see our basalts often, by gradation, losing their 

 own forms, but never assuming that of another variety; 

 and, in the last stage of evanescent form, we can trace an 

 eflbrt to preserve their own appropriate figure. This is 

 very observable in our columnar basalt, and in the long 

 horizontal prisms, of our whjm d3rkes. 



I can also trace something like a generic difference, 

 bet^v:een the varieties of our basalt: for some of them 

 have but one principle of construction, to wit, the ex- 

 ternal, visible forms;' into which, upon the slightest in- 

 spection, they appear to be divided l no internal con- 

 struction; the fracture irregular, and generally conchoi- 

 dal. The basalts of this class are, the columnar, the ii"- 

 regular prismatic, and the tabular. I have not been able 

 to discover subordinate forms, or an inttrnal construction, 

 in any of these basalts. 



Other varieties, on the contrary, are regularly arranged 

 internally; the large prism breaking into smaller, some- 

 times to a great degree of minuteness, as in the Port- 

 riish siliceous basalt. The coarse Portrush basalt, Avhose 



. o 2 prisms 



