LAY 



[249 ] 



LAV 



lopa or lauffen, to run, and is applied 

 to the melted and liquified matter, 

 discharged from the mouths of 

 volcanoes.) The matter which 

 flows in a fused, or melted, state 

 from a volcano. 



Lava, whatever be its chemical 

 composition, puts on very different 

 appearances, according to the cir- 

 cumstances which accompany its 

 consolidation, hence by some au- 

 thors it has been divided into 

 compact lava, cellular lava, and 

 cavernous lava. The mineral called 

 felspar forms, in general, more than 

 half of the mass of modern lavas. 

 When this is in great excess, lavas 

 are called trachytic ; when, on the 

 other hand, augite prevails, they 

 are called basaltic. Lavas occur 

 of an intermediate composition, 

 and these, from their colour, have 

 been called gray-stones. When 

 lava is observed as near as possible 

 to the point whence it issues, it is 

 found to be, for the most part, a 

 semi-fluid mass of the consistence 

 of honey, but occasionally so 

 liquid as to penetrate the fibre of 

 wood. It soon cools externally, 

 and consequently exhibits a rough 

 uneven surface ; but, from its 

 being a bad conductor of heat, 

 the internal mass remains liquid 

 long after that portion which 

 is exposed to the air has become 

 solid. That of 1822, some days 

 after it had been ejected, raised the 

 thermometer from 59 to 95, at a 

 distance of twelve feet ; at a dis- 

 tance of three feet, the temperature 

 greatly exceeded that of boiling 

 water. The temperature at which 

 lava continues in a state of fluidity 

 is sufficiently great to melt glass 

 and silver ; even stones are said to 

 have been fused when thrown into 

 the lava of Etna and Vesuvius. 

 The length of time during which 

 streams of lava retain their heat is 

 quite astonishing : the current of 

 lava which flowei from Etna in 



1669 retains a portion of it to the 

 present time. That which was 

 poured from Jorullo, in Mexico, in. 

 the year 1759, was found to retain 

 a high temperature half a century 

 afterwards. Sir W. Hamilton 

 lighted small pieces of wood in the 

 fissures of a current of Vesuvian 

 lava four years after it had been 

 ejected. The streams of lava often 

 become solid externally, even while 

 yet in motion, and their sides may 

 be compared to two rocky walls, 

 which are sometimes inclined at an 

 angle of 45. Of the immense 

 bodies of lava thrown out during 

 volcanic eruptions, few persons en- 

 tertain a just idea. Etna, which 

 rises upwards of 10,000 feet in 

 height, and embraces a circumfer- 

 ence of 180 miles, is composed en- 

 tirely of lava. " In the structure 

 of this mountain," says Dr. Dau- 

 beny, " every thing wears alike 

 the character of vastness." The 

 products of the eruptions of Vesu- 

 vius may be said almost to sink 

 into insignificance, when compared 

 with these coulees, some of which 

 are four or five miles in breadth, 

 fifteen in length, and from fifty to 

 one hundred feet in thickness. Still 

 the eruptions of Etna are nothing 

 when compared with that of Skap- 

 tar Jokul. " On the llth of June 

 Skaptar Jokul threw out a torrent 

 of lava which flowed down into 

 the river Skapta, and completely 

 dried it up. The channel of the 

 river was between high rocks, in 

 many places from four hundred to 

 six hundred feet in depth, and 

 near two hundred in breadth. Not 

 only did the lava fill up these great 

 defiles to the brink, but it over- 

 flowed the adjacent fields to a 

 considerable extent. The burning 

 flood, on issuing from the confined 

 rocky gorge, was then arrested for 

 some time by a deep lake, which 

 it entirely filled. The current 

 then again proceeded, and reaching 



