LAV 



[ 141 ] 



LEA 



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 

 distance 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 flowed 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. Ha- 

 milton 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 exter- 

 nally, 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 entertain a just idea. Etna, 

 which rises upwards of 10,000 feet in 

 height, and embraces a circumference of 

 180 miles, is composed entirely of lavas. 

 " In the structure of this mountain," says 

 Dr. Daubeny, " every thing wears alike 

 the character of vastness." The products 

 of the eruptions of Vesuvius 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 erup- 

 tions of Etna are nothing when compared 

 with that of Skaptar Jokul, the following 

 account of which is extracted from Mr. 

 Lyell's Principles of Geology. " 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 be- 

 tween 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 overflowed the adja- 

 cent fields to a considerable extent. The 

 burning flood, on issuing from the con- 

 fined rocky gorge, was then arrested for 

 some time by a deep lake, which it en- 

 tirely filled. The current then again pro- 

 ceeded, and reaching some ancient lava 

 full of subterraneous caverns, penetrated 

 and melted down part of it. On the 18th 

 of June, another ejection of liquid lava 

 rushed from the volcano, which flowed 

 down with amazing velocity over the sur- 



face of the first stream. After flowing 

 for several days, it was precipitated down 

 a tremendous cataract called Stapafoss, 

 where it filled a profound abyss, which 

 that great waterfall had been hollowing 

 out for ages, and again the fiery current 

 pursued its onward course. On the 3d of 

 August, fresh floods of lava still pouring 

 from the volcano, a new branch was sent 

 off in a different direction. "When the 

 fiery lake which filled up the lower por- 

 tion of the valley of the Skapta had been 

 augmented with new supplies, the lava 

 flowed up the course of the river to the 

 foot of the hills whence the Skapta takes 

 its rise. This eruption did not entirely 

 cease till the end of two years, and al- 

 though the population of Iceland did not 

 exceed fifty thousand, not fewer than 

 twenty villages were overwhelmed, besides 

 those inundated by water, and more than 

 nine thousand human beings perished, to- 

 gether with an immense number of cattle. 

 Of the two branches of liquid lava, which 

 flowed in nearly opposite directions, the 

 greater was fifty, and the lesser forty 

 miles in length. The extreme breadth 

 which the Skapta branch attained in the 

 low countries was from twelve to fifteen 

 miles, that of the other about seven. The 

 ordinary height of both currents was one 

 hundred feet, but in narrow defiles it 

 sometimes amounted to six hundred." 



LAU'MONITE. Diatomous zeolite. A mine- 

 ral thus named after its discoverer, Gillet 

 Laumont. It is of a white, or grayish- 

 white colour, sometimes tinged with red. 

 It occurs regularly crystallized, and in 

 distinct granular concretions. Its crystals 

 are four-sided prisms, slightly oblique, 

 sometimes terminated by diedral summits, 

 sometimes truncated on their lateral edges. 

 By exposure to the air, Laumonite disin- 

 tegrates, and is at length reduced to a 

 white powder. If, however, recent spe- 

 cimens be immersed for two or three hours 

 in a strong mucilage of gum, the action of 

 the atmosphere upon them, and their 

 efflorescence, will be prevented. Laumo- 

 nite consists of silica 52*0, alumina 21*20, 

 lime 10-50, and water about 14. It occurs 

 in secondary trap-rocks in France, Scot- 

 land, Iceland, and America. 



LA'ZULI. See Lapis lazuli. 



LA'ZULITE. A mineral of a light blue co- 

 lour, supposed by some mineralogists to 

 be a sub-species of lapis lazuli. 



LEAD. (Iced, Sax.) Lead is of a bluish- 

 grey colour, with considerable lustre, but 

 soon tarnishes by exposure to the atmo- 

 sphere. By friction this metal exhales a 

 peculiar, and somewhat disagreeable, 

 odour. Its specific gravity is 11 '35, or 

 nearly eleven and a-half times heavier 

 than water. It is soft and easily melted, 



