92 GEOLOGICAL MEMOIRS. 



form, these latter being generally 'enninated with a pear-shaped 

 knob, like what one sees at the extremity of a thread of glass that 

 has been melted. In this state the knob is of darker colour to- 

 wards the extremity, and quite black when of larger size than usual, 

 being then like the fragments of obsidian found detached in the ge- 

 neral mass. Detached threads terminated in a point, and many of 

 them exhibited distinctly under the microscope a number of strias 

 along the length of the thread. One might suppose this to indicate 

 crystalline structure, if it were not that drawn threads of glass 

 exhibit similar striations ; but at the same time I am not inclined 

 absolutely to deny the crystalline origin of the specimens in question, 

 though it seems to me more probable that they have resulted from 

 certain conditions of cooling after the mass has undergone fusion. 



This singular condition of obsidian is not however without par- 

 allel. Von Born, in the ' Catalogue des Fossiles de la Collection de 

 Madm^i^ E. de Raab ' (Vienna, 1790, i. 454), speaks of " a volcanic 

 glass in detached filaments, capillary, vitreous and of green colour, 

 from the Isle of Bourbon." These vitreous and flexible filaments 

 were thrown out during the eruption of 14th of May 1766. Haus- 

 mann also (Jahrb. 1837, p. 500) describes a similar product from 

 the same island, using an ingenious analogy in illustration. He says, 

 " An appearance consisting of a very loosely compacted confused 

 mass of extremely delicate glass threads is sometimes seen in glassy 

 slag, and has a similar origin to the so-called spun glass. It is 

 sometimes formed in the hearth of a blast-furnace, where the oppo- 

 sing currents of wind form and retain for a long time a number of 

 balls of slag in the hearth. Sometimes a somewhat similar result is 

 produced during a volcanic eruption, as happened in the island of 

 Bourbon in the year 1821, when there fell a shower of exceedingly 

 fine volcanic ashes consisting of delicate glass threads." 



I am informed by Professor Wiebel of Hamburg, that Captain 

 Wilken had also brought from Owhyhee numerous specimens of 

 obsidian, the broad crevices of which were filled up M'ith a similar 

 confused heap of particles ; a fact which is by no means inconsistent 

 with the explanation ofl^ered by Hausmann, since gases may here 

 have performed the part of the artificial blast. 



I have thought this short notice worthy of being communicated, 

 since volcanic products of this kind must always be rare; and the 

 analogous phaenomena from Bourbon are not known to me by per- 

 sonal observation. 



D. T. A. 



