242 Prof. J. W. Judd — On Marekanite and its Allies. 



whose memoir entitled "Chemische Untersuchung des Marekanits " * 

 was read before the Berlin Academy on the 15th May, 1812. 



Klaproth showed that the Marekanite-balls were sometimes almost 

 colourless and translucent, at others dark-coloured and opaque ; the 

 former he found to have a specific gravity of 2-365, the latter of 

 2-335. The composition of the two varieties he determined to be 

 as follows : — 



Transparent Variety. Opaque Variety. 





81-00 

 9-50 

 0-43 

 0-60 

 2-70 

 4-50 



0-50 ,, , 



76-50 





10-50 

 0-50 





1-00 

 2-70 

 4-50 



Undetermined constituent . . 



2-50 

 0-50 







99-13 98-70 



No more recent analyses of the rock would seem to have been 

 published. Klaproth further showed that while many of these glass 

 balls resist a tolerably smart blow with a hammer, yet when they 

 do yield they break up into a mass of fine particles, like unannealed 

 or toughened glass. He noticed, too, their singular behaviour when 

 heated, which will be described in the sequel. 



Klaproth's analyses show that Marekanite is an obsidian, and if 

 his separation of the soda and potash can be relied upon, which is 

 doubtful, we might conclude that it is a dacite- or quartz-andesite- 

 glass rather than a rhyolite- or quartz-trachyte-glass. 



With the exception of some observations by Erman 2 and Herter, 3 

 but little seems to have been added to our knowledge of Marekanite 

 since the time of Klaproth. 



Professor Bonney informs me that the late Professor W. H. Miller, 

 of Cambridge, told him that Marekanite-balls had been known to 

 explode spontaneously. Mr. J. Gregory some time ago communi- 

 cated to me the interesting fact that a lapidary who had attempted 

 to cut a section of a Marekanite-ball found that during the operation 

 it flew to pieces with a slight explosion. Kalkowsky states that 

 many of the Mai-ekanite-nodules. when struck with a hammer, de- 

 crepitate to dust. 4 



In these characters the balls of Marekanite appear to present 

 so many points of resemblance with the well-known "Kupert's 

 drops " that I thought it would be of interest to make a fuller 

 comparison of the natural and artificial substances. 



In the first place, the more transparent varieties of Marekanite, 

 when examined between crossed Nicols, were found to depolarize the 

 light in a very striking manner, giving rise to the well-known appear- 

 ance presented by unannealed glass or glass when subjected to 

 mechanical strain. Compared with the appearances presented by 



1 Berlin Abhandl. Physikal Klasse (1812—13), pp. 49—58. 



2 Archiv. f. d. "Wissensch. Kunde von Eussl. iii. (1843), p. 175. 



3 Zeitschr. d. d. Geolog. Ges. xv. (1863), p. 459. 

 i " Elemente der Lithologie " (1886), p. 80. 





