ORIGIN OF BASALT. S'S^ 



stituent materials are more completely blended than iri others: Facts and ob» 



I • t .1 ^1 • ^ ii. • I, j-f servatioiis ve- 



whith seems the same thmg as to say, there is much dit- spgctin-^ basalt 



ference in grain ; a great interval between the coarsest in the county 



and the finest. But all this is by insensible shades; "<> duceTta s'h^w 



siuch thing as drawing lines, by which we can mark the that it is not » 



varieties of tins fossil. Even where other differences are volcanic. 



most essential, between the varieties of basalt, inspection 



cannot be relied upon. For instance, the siliceous basalt, full 



of marine exuvias, passes, by gradation, from a grain as fine a"* 



jasper, until it becomes indistinguishable from, the Giant's. 



Causeway stone, and even coarser. 



If we look to nature for assistance, in classing the varietfes^ 

 of basalt, we will be no longer at a loss : we will find, she has. 

 impressed an indelible character on each variety of this 

 fossil; a specific figure, into which every stratum is divisible, 

 in its whole extent, being formed, as it were, by an ag- 

 ghitination of similar figures;* in the same stratum, all of 

 nearly the same degree of perfection ; but, when we com, 

 pare different strata, of the same variety, the perfection or 

 neatness of the work vaiies, until it passes into an amorphous 

 mass. 



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

 servation of the distinctive characters,^ of the different varieties- 

 of basalt, as she has done, to prevent confusion in the several 

 tribes of the animal and vegetable kingdoms. We see Oiir 

 basalts often, by gradation, losing their own forms, but 

 never assuming that of another variety; and, in the la«;i: 

 stage of evanescent form, we can trace an effort to preserve 

 their own appropriate figure. This is very observable in our 

 cokimnar basalt, and in the long horizontal prisms of our whyn 

 dykes. 



I can also trace something like a generic difference, be- 

 tween the varieties of our basalt: for some of them have 

 but one principle of construction, to wit, the external ri- 

 sible forms; into which, upon the slightest inspection, they 

 appear to be divided : no internal construction ; the frac- 

 ture irregular, and generally conchoidaL The basalts of 



this 



* I do not use the word similar in a strict mathematical sense; mean- 

 ing no more than a strong, general likeness, so decided, that the figure* 

 •f one variety cannot be mistaken for those of another. 



