OCR Text |
Show 346 EFFECTS OF DECOMPOSITION ON LAVA. · effect 1. s pro d uce d · In the lac va-currents of Central p1Cturesque f 1 v· r·ais in particular), the uppermost F 1. e (those o t le Iva ' . . h rati~Ocn often c1 01· ty Jce e t or more in thickness, IS a·n amo· rp ouds po ' . l .. 1 I'nto lava irregularly pnsmatlc; an , PassmO' c own war us ' . l l mass 0 d tl · there 1. s a 1c oun d a tt'on of reOo 'ular and vert1ca co umns, u· n the r t puasr, t f 1 . t ..,hich must have cooled most slowly. 0 t 1e curren " mB t at he lavas 1a s t men tl. o ned arc often one .h undred feet or muo re m. thi·C 1{ ness,. anc1 we cannot expect to dis.c over, tl1 1 e sahm .e henomenon m. t h e s1 1 al l o w currents of V esuvdm s, ha'1 th 1 oug Idt Pm ay be loo k ed f or .m mo dern streams in Icelan ' w lC •e xcee e:e n those o.f anci.e n t France in volume. Mr. Scf ro1p e mde n- t ons that m tl1 e cl 'IJur.s encirclinOo' the great crater od 't 1.e . mo ernd 1 e ' he sa' w many curren ts offering a columnar IVlSion, ladn con ' almost as regu1 a rl y prismatic as any ra. nges of the. o er some d h dd that in some the spherOidal concretionary basalts; an ela. s, . le was equally conspicuous. Brieslakt structure on a atge sea ' 7 h' h . 1 . f, ' us that in the siliceous lava of 173 ' w IC con-a ~o m or~s ' ite and cr stals of felspar, he found very tams augt.t e, le·u c q' uany neYar T ort. e del Greco •· which obser-regu. lar .p nsmsfi m ead by mo d ern au th or"iti"e :·s ++. The decompo· v;~•:n o~s .:: ~;the felspathic lavas, either by simple ~ea~e:~ 1 IO aseous emanations, converts them from a ar o mg, or by g th t they no lonO'er retain the smallest a soft clayey state, so a from = state of fusion. The resemblance to rocks cooled down n and muriatic acid which exhalations of sulph~lrettleld hfydrogt~e Solfatara also produce d. d contmua Y rom ' h are . Isengl age . on t h e trac h y t e of that extinct volcano: bt de cunou. s ch 1" angeds d becomes porous, fissile ' and honeycom e ' rock IS w Itene an bl . t a white siliceous powder~· . 1 tl it crum es m o . . till at eng l . osed of concentrlC lammre, Numerous globular concretiOns, comp_ this decomposed rock!!· arc also formed by th~. sa~~~~:P;~~~;:~rean Fields and the vol· They who have visite o f the manv pro· . f s· '1 and who are aware o ~ canic r.e giOn o 1C1 y' hich ]. O'neOUS roc ks of the roost modern blemattcal appearanc~s w f o d position cannot but be origin assume, especially a ter ecom ' V dans la Campam·e , to m e • i ' P· 201. "' Journ. of Sci., vol. xv., p.l77.. t. oJy. fSci. No.xviii., Oct.l828. M t Vesuvms Edm. ourn. o ' t Mr. Forbes, on oun ' 169 + 1.. Daubeny on Volcanos, P· · 'i .. 346 G 1 Trans second series, vol. n., V· ' 11 Scrope, eo • '' . VESUVIAN MINERALS. 347 astonished at the confidence with which the contending Nep· tlmists and V ulcanists in the last century dogmatized on the igneous or aqueous origin of certain rocks of the remotest antiquity. Instead of having laboured to acquire an accurate acquaintance with the aspect of known volcanic rocks, and the transmutations which they undergo subsequently to their first consolidation, the adherents of both parties seem. either to have considered themselves born with an intuitive knowledge of the effects of volcanic operations, or to have assumed that they required no other analogies than those which a laboratory and furnace might supply. l7esuvian Mine·rals.-A great variety of minerals are found in the lavas of Vesuvius and Somma; for there are so many common to both, that it is unnecessary to separate them. Augite, leucite, felspar, mica, olivine, and sulphur, are most abundant. It is an extraordinary fact, that, in an area of three square miles round Vesuvius, a greater number of simple minerals haye been found than in any spot of the same dimensions on the surface of the globe. Hauy only enumerated three hundred and eighty species of simple minerals as known to him, and no less than eighty-two had been found on Vesuvius before the ·end of the year 189l8 *. Many of these are peculiar to that locality. Some mineralogists have conjectured that the greater ,part of these were not of Vesuvian origin, but thrown up in fragments from some older formation, through which the gaseous explosions burst. But none of the older rocks in Italy, or elsewhere, contain such an assemblage of mineral products; and the hypothesis seems to have been prompted by a disinclination to admit that, in times so recent in the earth's history, the laboratory of Nature could have been so prolific in the creation of new and rare compounds. Had Vesuvius been a volcano of high antiquity, formed when Nature Wanton 'd as in her prime, and play'd at will Her virgin fancies, it would have been readily admitted that these, or a much greater variety of substances, had been sublimed in the crevices of lava, just as several new earthy and metallic compounds are known to have been produced by fumeroles, since the * Monticelli and Covelli1 Prodrom. della Mineral. Vesuv. |