PUMICE. — OBSIDIAN. 



279 



that no pores or filaments are visible to the eye ; when viewed with 

 a lens, they appear like an accumulation of small flakes of ice. 

 Though apparently compact, they swim on water. Other pumices 

 contain pores and cavities, and are composed of shining white fila- 

 ments. By a long continued heat, pumice-stone melts into a vitreous 

 semi-transparent mass, in which a number of small crystals of whhe 

 felspar are seen. Black or dark-coloured pumice is more uncommon, 

 Humboldt says, he has seen black pumice in which augite and horn- 

 blende may be recognized ; he is inclined to think that such substan- 

 ces owe their origin to basaltic lavas, which, by intense heat, have 

 assumed a capillary or fibrous form. 



Immense quantities of pumice are sometimes thrown up by sub- 

 marine volcanoes. It has been seen floating upon the sea over a 

 space of three hundred miles, at a great distance from any known 

 volcano : from hence it may be inferred, that submarine volcanoes 

 sometimes break out at such vast depths under the ocean, that none 

 of their products reach the surface, except such as are lighter than 

 water. 



Obsidian, or volcanic glass, so nearly resembles lumps of black glass, 

 that they can scarcely be distinguished by the unpractised observer. 

 Its broken surface is smooth, conchoidal, and shining : the most com- 

 mon colour of obsidian is a velvet black. The thinner pieces are 

 translucent. It is harder than glass and strikes fire with steel. It is 

 common in the neighbourhood of volcanoes, and in some basaltic for- 

 mations. The obsidian accompanying basalt, contains a large por- 

 tion of augite, and melts into a black glass, as before mentioned ; in 

 other respects, its mineral characters are the same as those of ob- 

 sidian from trachyte. In Lipari, one of the volcanic isles, the moun- 

 tain de la Castagna, according to Spallanzani, is composed wholly 

 of volcanic glass, which appears to have flowed in successive currents^ 

 like streams of water, falling with a rapid descent, and suddenly fro- 

 zen. This glass is sometimes compact, and sometimes porous and 

 spongy. Numerous veins of obsidian are said to intersect the cone 

 of Mount Vesuvius, and serve as a cement, to keep together the 

 loose materials of which it is composed. 



On the elevated plain which surrounds the conical peak of Tone- 

 rifFe, there are masses of obsidian, which graduates into pitchstonOj 

 containing crystals of white felspar. On the south-west side of the 

 peak, there is a stream of vitreous lava or obsidian, several miles in 

 length. Colonel Imrie describes a current of lava in the island of 

 Felicuda, intermixed with obsidian, which had been flowing with itj 

 and now forms part of the congealed stream. " In some parts the 

 obsidian is seen losing its brilliancy, and passing into granular lava^ 

 which becomes similar in colour, fracture, and texture, to the other 

 parts of the stream. Where the obsidian appears in a state of per- 

 fect glass, it is very near to where it has been first ejected from the 

 side of the crater, and in a situation where it must have undergone a 



