OF MONTE SOMMA AND VESUVIUS. 99 
able for the perfection of the constituent minerals, especially the 
leucite, which often attains very large dimensions. 
The primary or pumiceous materials consist of fragments very 
variable in size, and forming about half the deposit, the rest being 
made up of leucilitic lapilli and foreign rocks. ‘These latter occur in 
considerable abundance, and sometimes as moderately large blocks. 
Their origin is, no doubt, the same as that of those of the pre- 
Plinian eruption, and to the casual observer they appear to be similar. 
A careful examination of a large quantity of them, and their mineral 
contents, shows them to be richer in some, and at the same time to 
abound in certain species, at the expense of others common in similar 
materials of earlier date. Thus these minerals, common in the pre- 
Plinian eruption, are replaced to a large extent by those given 
in the second column. 
Pre-Plinian * , Puase VII., Period 6. 
Wyle voor hoes eine cs otha esd ncerentreeet Mizzonite and sarcolite. 
Nepheline. 
BeTRCONA HNO Nets Pa serie vs ies ek ots ae acta eset ec Hnwyne, 
Lapis lazuli. 
Bag MONOM ED sake caacetansecactiec ntsc. ortitiaeisaee Spinel (black). 
Be AMUN hee a ede ee een ot i ciktie witente Seine Se Leucite. 
PAM MTIDOLES Lute sche thebeteeaes iceasdacaceass Pyroxene. 
This list shows a certain relation between the two sets of minerals, 
but it is difficult to draw any definite conclusion therefrom, until our 
knowledge of the chemical relations of these minerals to the variety 
of igneous ejectamenta is much more extended. Nevertheless the 
great abundance of hauyne, lapis lazuli, and nepheline in foreign 
blocks, associated with a primary material rich in well-crystallized 
leucite, is worth thinking over when we remember the close rela- 
tionship of these mineral species. Their comparative rarity in the 
other deposit, where leucite is almost, if not entirely, absent, adds 
still more force to the fact. Although such an interesting relation- 
ship may light up in our thoughts a slight glimmer of some important 
speculations, it is safer to be patient until we have a more solid 
foundation to build upon, and our materials are more clearly worked 
out and arranged. Before leaving the subject, perhaps one other fact 
may besimply mentioned with regard to the replacement of pyroxene 
and spinel. Messrs. Fouqué and Lévy7, in making a nephelinite, 
fused 3 parts of nepheline and 1:3 of augite together, and obtained 
the same minerals. When they diminished the dose of augite, they 
got fine crystals of nepheline, little crystals of spinel, and brown 
dodecahedra of garnet (melanite). 
* These are the general observations of the author, who has broken up many 
hundreds of ejected blocks from both eruptions. It must be remarked, how- 
ever, that there is only a prevalence; for all these species may be obtained from 
either deposit with more or less difficulty. No accurate statement can be made; 
the above, therefore, is only the expression of an opinion derived by experience, 
and confirms the mineral-hunters’ dicta as to certain species being found in 
certain localities. 
T ‘Synthése des minéraux et des roches,’ p. 60. 
vet bee 
