Lavis — On the Structure of Rocks. 143 



minerals one by one and compare their occurrence in nature with 

 their reproduction artificially. 



Peridote was obtained, 1 amongst other methods, by remit, at a 

 white-red heat, of the elements of a basalt, exactly identical in all 

 characters with what occurs in nature. This mineral occurs 

 naturally in two forms. The first are irregular nodular masses, 

 found as bombs, or entirely enveloped in the lava. From their 

 large size they must have required a long time to crystallize, which 

 took place in all probability before the extrusion of the magma. 

 They, no doubt, resulted in some cases by actual crystallization 

 from the igneous matter ; but, I believe, by far the larger part 

 are nothing more than a very advanced metamorphism of a 

 dolomite; for amongst the ejected blocks of Monte Somma or 

 Eoccamonfina we may obtain all gradation between the original 

 sedimentary rocks and these masses of pure olivine. The most 

 common form, in a petrological point of view, is the disseminated 

 grains that often go to make up a rock. These are seen to be 

 nearly always one of the first conversions of the amorphous paste 

 into formed material. Yet the actual conditions suitable to its 

 crystallization are not quite clear ; for we find lavas ejected from 

 the same volcano abound with it sometimes, and at others it is 

 quite difficult to find. So far as my observations go, it favours the 

 basic rocks of fine-grained structure, and especially those that 

 have cooled quickly from a very high temperature, although it 

 seems capable of increasing in size during slow cooling from a 

 very high temperature, in consequence of the lava stream being 

 very deep. This is the case with some very coarse lavas of 

 Vesuvius, such as that of Pompeii and Cisterna, which contain 

 some crystals a centimeter long. 



Amphibole. — This mineral has baffled all the attempts of the 

 chemist to prepare it artificially otherwise than as a sublimation. 

 Whenever its elements were fused separately, or a complex mix- 

 ture, was fused, the only product was its ally — pyroxene. Our 

 entire acquaintance with amphibole indicates it as a mineral crys- 

 tallized under pressure, and probably from an aquiferous magma. 

 Its continual occurrence in syenites and allied rocks show it to be 

 easily crystallizable under the conditions which these rocks came 



1 Fouque and M. Levy. Bull. Soc. Min. 1881, t. iv., p. 275. 



