246 Prof. J. W. Jucld — On Marekanite audits Allies. 



consequence of its presence, specimens of the rock exhibit not the 

 vitreous lustre of obsidian, but the duller lustre of pitchstone. 1 



At the time when I made this observation, I thought that the 

 remarkable change in the surface of the rock might be due to 

 chemical action, and was the result of some kind of exudation on the 

 freshly-formed surfaces. But subsequent study of the surfaces with 

 the microscope, and discussion of the case with others, including 

 Prof. Bonney and also Dr. Guthrie, who has paid much attention to the 

 subject of the fracture of colloids, 2 has convinced me that the change 

 is really mechanical, and one of a very peculiar kind. 



Pitchstone Vein at Chiaja di Luna, Island of Ponza. 

 Experimenting with colloid substances, like plate-glass and 

 Canada-balsam, I found that when relief from intense strain took 

 place by the breaking of a mass which had been bent, till it could 

 no longer resist the force applied, the fractured surfaces show a 

 dull appearance, which under the microscope is seen to be caused 

 by the wrinkling up of a thin film on the suddenly liberated 

 surfaces. These cases differ from the one described from 

 the Ponza Islands, in the circumstance of the appearance of the 

 wrinkled film apparently instantaneously, instead of after an ap- 

 preciable interval of time. But it is, I think, consistent with well- 

 recognized physical principles that the recovery of particles from 

 a condition of strain, which has been long maintained, should take 

 place more slowly than that which follows from a strain of short 

 duration. 



The study of cases which we have been considering all point to 

 the conclusion that, besides the local strains so well known to occur 



1 Geol. Mag. Dec. II. Vol. II. (1875), p. 308. 



2 See Phil. Mag. ser. 5, vol. viii. p. 25. 



