Chap. 17.] 'Bajaltes. 107 



fometimcs is found in large columns with convex 

 and concave articulations, fo as to referable an artifi- 

 cial ftrudhire : of this kind are the bafaltic pillars in 

 Ireland, called the Giant's Caufeway. 



Bafaltes has always, till very lately, been confidered 

 as a volcanic product ; and in corroboration of this 

 opinion it is -aiTerted that glafs, in cooling, has been 

 known to affume the regular bafaltic form. Sir Wil- 

 liam Hamilton remarked, both in Sicily and Naples, 

 tnat fuch lavas as have run into the fea are either 

 formed into regular bafaltes, or have a great tendency 

 to that form. Mr. Kirwan, however, in the laft edition 

 of his Elements of Mineralogy, takes confiderable pains 

 to controvert 'the common opinion. His principal ar- 

 guments to prove that bafaltes are formed by water, 

 and not by fire, are the clofenels of their textures, 

 fince they are quite free from the cavities which are 

 numerous in all other volcanic products, and their 

 containing mild calcareous earth, which could not have 

 been the cafe if they had been fubjected to fuch a 

 degree of heat as to have reduced them to a ftate of 

 fufion. Bafaltes alfo contain zeolites in fome in- 

 ftances, which muft have loft the water combined with 

 them, if they had been fubjected to much heat. In 

 bafaltes there is no appearance of vitrification, which 

 they muft have had if they had cryftallized from a 

 ftate of fufion. Bafaltes are alfo frequently found, not 

 only at a diftance from volcanoes, but mixed and fur^ 

 rounded with ftrata, which have evidently derived 

 their origin from water. For inftance, their fubftance 

 fometimes pafles gradually into argillites and fand- 

 ftones. Mr. Kirwan feems at a lofs to account for 

 the peculiar fhape of bafaltes ; but this is equally dif- 

 ficult on the principles of either theory. He mentions 

 an inftance, however, of a ftone cracking into pieces 



of 



