Prof. J. W. Jucld — On Marekanite and its Allies. 245 



which vary in size from a pea to a fist, constitute the well-known 

 bodies usually called Marekanite. 



A number of rocks have been described from other localities, 

 which in their peculiar characters or mode of occurrence appear to 

 closely resemble Marekanite. 



In 1844 Damour gave an account of a specimen of obsidian, 1 

 which was said to have been obtained from India, though its locality 

 could not be more precisely determined. It was spheroidal in form, 

 and about 2^ inches in diameter ; its colour was black by reflected 

 light and bottle-green by transmitted light, and it had a specific 

 gravity of 2-47. Unlike Marekanite, it was found to melt slowly 

 without expansion, and to form a colourless glass. This ball of 

 obsidian, while being cut by a lapidary, was found, when sawn through 

 to the extent of nearly two-thirds of its diameter, to emit a kind of 

 hissing sound, and this was followed by a strong detonation, the mass 

 being at the same time broken up into fragments, which were thrown 

 in all directions with considerable violence. The fragments when 

 collected were found to show a radiated structure like that seen in 

 the Marcasite-nodules of the Chalk. 



The obsidian described by Damour was found by him to have the 

 following composition, so that it must be regarded as a dacite-glass. 



Silica 70-34 



Alumina 8 - 63 



Ferrous oxide ... ... ... 10-52 



Manganous oxide 0-32 



Lime ... ... ... ... 4-56 



Magnesia 1-67 



Soda 3-34 



99-38 



It is said not to have lost weight on ignition. 



In 1824 the late Mr. Poulett Scrope described 2 an interesting 

 vein of green, porphyritic pitchstone with contorted banding, which 

 occurs at the Chiaja di Luna in the Island of Ponza ; he showed that 

 the rock is made up of columns, each of which incloses a number of 

 globiform masses that break up concentrically like gigantic onions. 

 Mr. Scrope's drawing of this remarkable rock-mass is reproduced in 

 the accompanying woodcut (see page 246). 



In 1874 I had an opportunity of visiting this very interesting spot, 

 and noticed a remarkable property exhibited by this green rock ; 

 when struck with a hammer, it broke up with curved planes of 

 fracture parallel to the central nuclei; when freshly broken, the 

 surfaces showed a perfectly vitreous lustre, but within a few seconds 

 a most remarkable change was seen to take place over the glassy 

 surfaces thus exposed. They appeared as though breathed upon and 

 were gradually overspread by a whitish film, that looked like the bloom 

 upon a peach ; this white film was found to be permanent, and in 



1 Comptes rendus, vol. xviii. (1844), p. 46. 



2 Trans. Geol. Soc. 2nd ser. vol. ii. p. 205, plate 24. See also Volcanoes, 2nd 

 ed. p. 105. 



