TRANSACTIONS OF THE SECTIONS. 121 



water, liydrocliloric acid, sulphuric acid, &c. exhaled bj' the er<ater3 and by the 

 streams of lava are not only an acconipanying phenomenon in the production of 

 volcanic rocks and mineral ag-gregates, but that they are essentially cooperating 

 agents in their origination. If once we succeed in proving and explaining the 

 origin of minerals through vapours, or by cooperation of vapours, then will the key 

 to many a problem relating to the plutouic rocks and their mineral veins be 

 found. 



A block of lava ejected by Vesuvius in its last eruption is very instructive, as 

 throwing light on the formation of minerals by volcanic ■\'apours. This block 

 shows that in its interior small and fine crystals of pyroxene, mica, sodalite, ha3ma- 

 tite (specular iron), and magnetite were formed, whiht at the same time in the peri- 

 pheric zone the pyroxene was melting and the leucite being destroyed by the vol- 

 canic heat. 



Originally our block of lava had doubtless the appearance of so many varieties of 

 lav'a from the dykes of the Somma. The character of the "lava antica " is well 

 known to those acquainted with Vesuvius. The original nature of our rock is in- 

 dicated by thickly agglomerated leucite of sizes up to 3 millims. (scarce), green 

 pyroxene as large as 5 millims., and a magma with a great many cavities. Our frag- 

 ment shows the constitution of the outside as well as of the interior. It is enclosed 

 in a covering of black lava a few millims. thick, which is full of bladders on the sur- 

 face, towards the interior dense and melted like obsidian. In this crust it is clear 

 that some parts of the ancient lava, for instance the pyroxene, were melted with 

 the new lava, in the midst of which our block was floating in the depths of the 

 crater before the eruption. Though in the periplieric zone there is nothing to be 

 scon of pyroxene, the lencite-crystals have oeen destroyed, but not melted ; they 

 rise out of the black scoriaceous matter as white grains soaked through, as it were, 

 by the grey melted mass. To this external zone another succeeds, lO-lo millims. in 

 thickness, in which the rock is firm and compact, find nothing is to be seen of new 

 mineral formations. At this zone the pyroxene-crystals are likewise melted, and the 

 cavities of the rock are filled with the melted mass penetrating from the outside. 

 How could the htematite have been formed so as to escape being fused down with the 

 igneous silicate ? At a distance of 12-15 millims. from the periphery the pyroxene 

 is not melted, at least not totally, to glassy drops ; and it is here that, naturally 

 without distinct limits, the inner part of the block commences in which the new 

 formations have taken place. The cavities here are not filled with a melted mass, 

 but covered with small delicate crystals that sparkle brightly and are in marked 

 contrast with the dense melted magma of the peripheric zone. The sparkling 

 covering of the little geodes consists principally of hrematite and reddish-yellow 

 pyroxene. In some cavities the sparkling is due to plates of ha?matite, in others 

 nearly entirely to reddish -yellow pyroxene. Most of them show both minerals 

 together and in intimate mixture. Both minerals, haematite and pj'roxene, 

 appear not only in the cavities, but also in the smallest fissures and holes of the 

 general mass, and even of the leucite. Studied by a lens, the rock is seen every- 

 where to exhibit minute shining points of black hajmatite and red pyroxene. The 

 little pyroxene-crystals, appearing here as new formations, resemble in colour and 

 hal)it exactly those crystals which seven years ago I found implanted on the 

 volcanic hrematite of the spent fumarole in a scoriaceous hill near our lake of 

 Laach, and which I announced as the first decided proof of a silicate originated by 

 sublimation. Magnetite in small quantities, but in the same circumstances as 

 pyroxene, is also found in our rock. By close observation of the little cavities one 

 may also observe a fourth mineral in white crystals of pearly lustre, formed like- 

 wise by sublimation. It was somewhat difficult to determine that mineral ; it has, 

 however, been done with great certainty : it is sodalite, 



The formation of haematite by volcanic snblimalion, formerly a riddle, is now 

 quite understood ; but the chemical process, how silicates are formed by sublima- 

 tion, is not as clearly understood even now. Chemistry will succeed in explaining 

 this geological fact, as it has done with the A'olcanic haematite. 



Even now we can settle with precision that M\ater and chloride of sodium effect 

 principally the origin of silicates bj' sublimation, as is the case with the volcanic 

 hfematite. Certainly it is not accid(;ntally that we find sodalite, the silicate most 



