6z Experiments m Whinjtciie and Lava. 



feems to hare entered readily into fufion, along with the whinftone, but to have kept fc- 

 parate from it, as oil feparates from water through which it has been difFufed, thus giving 

 rife to the fpherical form, which the nodules of calcareous fpar generally exhibit with 

 more or lefs regularity •. 



This circumftance accounts for an appearance which has mifled fome of the early ob- 

 fervers of our minerals. Many whinftone rocks externally refemble very porous lavas, but 

 when broken are always found to be quite compadl internally, and to contain numerous 

 round nodules of calcareous fpar. Near the furface, the nodules, being waflied out by 

 rain, have left the cavities which have given rife to this deception. The fpherical form of 

 the air holes in lavas, and of the nodules of calcareous fpar in whins, feems to have been 

 produced by a caufe common to both, the mutual repulfion of two fluids intermixed, but 

 not dlfpofed to unite. 



It muft be owned, that this theory of calcareous fpar is as yet hypothetical ; but it is 

 fupported by ftrong analogy, and promifes to be of fervice, by leading to decifivc experi- 

 ments and obfervations. I cannot help believing, that, by a careful examination of the 

 volcanic countries, fadts may yet be difcovered which will throw light on this fubjedt. In 

 order to promote and direft fuch refearches, I fhall beg leave to ftate fome obfervations 

 which I made in thofe countries in 1785, before I was attached to any fyftem of geology. 



It is generally fuppofed, that fome lavas of JEtna contain calcareous fpar and zeolite ; 

 but this I conceive to be a miftake. It is true, as I have feen, that many rocks of ^tna 

 contain thefe fubftances in abundance*, but in my opinion thefe rocks are no lavas, but 

 have flowed fubterraneoufly like our whins, and are the fame with them in every refpeft. 

 A particular diftrift of ^tna, comprehending the Cyclopian Ifles, and the country round 

 La Trezza, and the caftle of Jaci, is decidedly of this defcription ; and veftiges of the 

 fame kind occur in other parts of the mountain. In one place foflTile coal has been found, 

 and in another we faw marine (hells. In the neighbourhood of Bronte we obferved a high 

 ridge formed of ftrata of fandftone and limeftone, partly overflowed and concealed by re- 

 cent lavas, but fo placed as to render it evident that its continuation formed no inconfide- 

 rable part of the mountain. Thus, ^tna being compofed, partly of the fubterranean, and 

 partly of the external produ£lions of fire, may be expe£led to afford numberlefs opportu- 

 ntties of purfuing the comparifon between thefe two clafies t- 



A moft interefting fcene for fuch a comparifon occurs likewife on Vefuvius. The hif- 

 tory of this volcano is fimpler than that of JExnz, for it has been evidently formed, with all 

 its appendages, by the continued aftion of external eruptions, which have raifed it, at 

 fome remote period, from the bottom of a fea, occupying all the Campi Phlegrjei, and 

 walhing the furrounding Appenines. The whole volcano feems once to have confifted of 



* The modifications of the aSion of heat, occafioned by preflure, which have been taken into account 

 by no geologift but Dr. Hutton, diftinguiili his theory from all other igneous theories. 



+ M. Doloinieu has obferved this diftinflioi^, but fuppofes that the maffes which we conceive to have 

 flowed fubterraneoufly were erupted at the bottom of the lea. 



a Angle 



