144 Scientific Proceedings, Royal Dublin Society. 



into existence. I have found it in basic pumices continually 

 accompanying orthoclase (of which we shall speak next) in the 

 more vitreous and early stage of the explosive eruption ejecta- 

 menta of Vesuvius, Roccamonfina, &c. In the later stages of the 

 same eruption it does not appear to have increased in size or 

 abundance, whilst it is often enveloped in pyroxene ; and this 

 latter species is spread throughout the mass, increasing in propor- 

 tion as the rock approaches the lava type. This is the more 

 remarkable, because we know that amphibole is more fusible than 

 pyroxene ; whereas, if we exclude change of pressure, &c, it 

 should have crystallized later. This fact alone is quite sufficient 

 to disprove any relationship between the fusing-point of a mine- 

 ral and its order of crystallization. Where amphibole is found 

 in a lava, we have evidence that it existed as such before the 

 eruption of that material. It is not at all an uncommon mineral 

 lining vesicular cavities ; but it there shows itself to have been 

 deposited by sublimation, which is borne out by its discovery under 

 similar conditions in some furnace scorias. 1 



Orthoclastic felspar was obtained by M. Stanislas Meunier 2 

 by fusion and subsequent recuit of acid rocks. The product, 

 however, only consisted of crystalline concretions, having the 

 composition of orthoclase. Microliths only rewarded the efforts 

 of Messrs. Fouque and M. Levy 3 after a long recuit of eight 

 days. These facts are thoroughly borne out by the basic pumices. 

 Those that were cooled very rapidly in the first eruptive stage 

 exhibit large, well-formed crystals of sanidine associated with 

 amphibole, showing the similar conditions under which the two 

 minerals were formed. 4 In the latter stage of these eruptions the 

 large crystals have not increased in number or size (?) ; but from 

 the slower cooling a few microliths have formed. Another proof 

 is to be found in the occurrence of fragments of a porphyritic 

 rock, which is only the pumice magma that, in some outlying- 

 fissure, has cooled under pressure, and in some cases undergone 

 secondary alteration. This shows the sanidine crystals still larger 



i M. L. Bourgeois, Op. cit., p. 119. 



2 Comptes rendus, 1880, t, xc, p. 1009. 



3 Comptes rendus, 1878, t. lxxxvii., p. 700. 



4 Whether these are really orthoclastic is generally a very difficult matter to deter- 

 mine. 



