Remarhs on Volcanoes. 279 



computed; and hence concludes, that the products of volcanic 

 action in Iceland, are derived from two independent foci. 



But the most interesting part of his researches relates to 

 the changes which have been wrought upon these materials 

 by causes of subsequent operation. 



Few of the friends I see around me are old enough to have 

 witnessed the contests which for many years were waged 

 with so much fury between the advocates of the igneous and 

 aquous origin of basalt. 



In this controversy much stress, I recollect, was laid by the 

 Wernerians on the characters of trap tuff, which, it was con- 

 tended, could by no means admit of being referred to the 

 action of heat, whilst its passage into trap rocks rendered it 

 difficult to ascribe to the one an origin which was denied to 

 the other. 



Now, Professor Bunsen has, in the first place, beautifully 

 shewn that the species of tuff which prevails in Iceland, and 

 which is also abundant in Sicily, as is implied by its name 

 Palagonite, derived from the village of Palagonia, at the base 

 of Etna, possesses such a chemical composition as identifies 

 it with the pyroxenic rock of the neighbourhood. 



He has also succeeded in explaining those differences in 

 structure and in appearance, which, in spite of this corre- 

 spondence in the nature of its constituents, stamp it as a dis- 

 tinct mineral ; having traced such alterations to the operation, 

 not indeed of water alone, but of an alkali or an alkaline earth, 

 containing just so much water as to exist in the condition of 

 a hydrate, formed in either case by the influence of a tem- 

 perature equal to that of ignition. 



The Professor states, that he has actually succeeded in 

 converting basalt into palagonitic tuff, by mixing it in a state 

 of fine powder with thirteen times its weight of slaked lime, 

 or of potass. 



Thus, the very alkali, which may have been sublimed from 

 some internal focus of igneous action, might, if water were 

 also present, have been instrumental in converting an ordi- 

 nary pyroxenic rock into palagonite under the influence of 

 heat. 



Another difficulty which beset the Huttonian theory, arose 



