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Show 396 MINERAI. COMPOSlT roN OF' LAVAS. . . . nat excess lavas are caUed 1 When 1t 1s m gr" ' modern a vas. . . 1 7roxene) predominates, they are trachyt.ic; w1h .e n aunguttte ](or l)f c.omJ10Sition precisely inter- avas 0 , termed basa tiC. l . . lour have been called gray- . · nd from t JCII co · · medtate, occm' a d f quartz characterizes the gramt1c stones. A gr~at abun ance o rall considered by geologists and other ancient. r?cks,lno.w getn1eat r~ineral, which is nothing f . ongm w Jet cas 1 11 l ns o Jgncous ' 11" d . s rare in recent lavas, a t 1oug 1 more than silex crysta Ize.' 1. 'ti'on Hornblende, which J 1 · to their compost · silex enters argc Y m . l , 's rare in modern lava, nor . . ancient roc <.s, I ' . • IS so common m . . ,1 f any :J.O'e in which augtte does it enter largely mto llO~;s llo in so~e recent trachytes, abounds. :Mica occurs 1P enti u ·~e is in excess. We must . l present w tcre augt f but IS rare y to refer too hastily to a difference ~ era, beware, however, not . t tl bclon()' to the different circum .. characters which ~lay' m ;o~1~~ts of fi~e originate. stances under w1uch fthel p . ous rocks of our own times, we Wl speak o t 1c JO'ne · le1n tw e 1l portw. n w1 uI ?c 1 1 J1pens in violent eruptions to 1 la Vv mean t la sma . fl . d t the surface of the earth. e be forced up by elastic uli s o: and lava which cool in the 1 11 d to the sane ' sconre, , ' 1 d mere y a u e bt . access to that which is congea e open a1· r; b u t we cannot o am · 1 d 1 ndred or many t 1ousan 1 tre of many IU ' . under t 1e presst . d d sec in the dikes of Vesuvms 1 We may m ee ' ntmosp 1eres. ' 1. 'd tate under a pressure , 'd t d from a 1qm s ' ' · rocks conso•I a e . f 1· nd the rock so formed 1s t1 sand feet o ava, a · of l)erhaps a 'lOU ' ncific O"ravity than ordmary 11. nd of greater sp"' b 1 1 1 more crysta me a f melted matter raised above t le eve lavas. But the column o . f Vesuvius must be more than of the sea dtuing an eru~twn o l . than ten thousand feet d ~ t . 1 helo·ht anc mme ' 1 three thousan lee u t"> ' miles deep may be tie in Etna; and we kno~v notbhow manJ1e mountain and those 1 . 1 mmumcate etween 1 {! r ducts w uc 1 co ' f b . 'nO' matter which supp y o subterranean ]a <l . es or .s eas. o b 'mnOm' e x0h austed, the same vo 1c a nic thousands of ycar.s, Wllthout ~f hot. vapours from many cra~ersl ents The continua escape . atld the chemtca v . l b t en eruptwns, 1 durin()' the interva . e we JO • • sin the fumeroles of vo-o l . 1 gom()' on lot age . heat chan(l'es w nc 1 nre o 1 . JO 1. I'etain their mtensc o 1 t tl vo came .oc ' 1 canos, prove t la Ie e it to be otherwise; for as avai constantly' nor ~n· :e st~l;~~t~ess require many .years. to. co~f currents of mo el ~ e 't suppose the great reservmrs down in the open air, we mus NUMlllm OF VOLCANIC ERUPTIONS. 307 me~ted. matter at vast depths in the nether regions to preserve their high temperature and fluidity for thousands of years. During the last century, about fifty eruptions are recorded of the five European volcanos, Vesuvius, Etna, Volcano, Santorin, and Iceland, but many beneath the sea in the Grecian Archipelago and near Iceland may doubtless have passed unnoticed. If some of them produced no lava, others on the contrary, like that of Skaptar Jokul in 1783, poured out melted matter for five or six years consecutively, which cases, being reckoned as single eruptions, will compensate for those of inferior strength. Now, if we consider the active volcanos of Europe to constitute about a fortieth part of those already known on the globe, and calculate, that, one with another, they are about equal in activity to the burning mountains in other districts, we may then compute that there happen on the earth about two thousand erupTions in the course of a century, or about twenty every year. However inconsiderable, therefore, may be the superficial rocks which the operations of fil'e produce on the surface, we must suppose the subterranean changes now constantly in pl'ogress to be on the grandest scale. The loftiest volcanic cones must be as insignificant, when contrasted to the products of fire in the nether regions, as arc the deposits formed in shallow estuaries when compared to submarine formations accumulating in the abysses of the ocean. In regard to the chamcters of these volcanic rocks, formed in our own times in the bowels of the earth, whether in rents and caverns, or by the cooling of Jakes of melted lava, we may safely infer that the rocks are heavier and less pomus than true lavas, and more crystalline, although composed of the same mineral ingredients. As the hardest crystals produced artificially in the laboratory, require the longest time for their formation, so we must suppose that where the cooling down of melted matter takes place by insensible degrees, in the course of ages a variety of minerals will be produced far hat·der than any formed by natural processes within the short period of human observation. These subterranean volcanic rocks, moreover, cannot be stratified in the same manner as sedimentary deposits from water, although it is evident that when great masses consolidate from a state of fusion, they may separate into natural divisions ; for this is seen to be the case in many lava- |