OF THE NORTH-EAST OF FIFE. 429 



striking flow-structure. ISTot only are these minuter elements of the 

 rock arranged in irregular parallel bands, but they are crowded in 

 front and along the sides of the porphyritic crystals, trailing off 

 behind them. 



Still more striking and beautiful is the iierlitic structure of this 

 remarkable rock. I know of no glass, ancient or modern, which 

 exhibits this structure in greater perfection. In manj^ parts of the 

 mass this perlitic structure is curiously complex, the larger spheres 

 into which the mass breaks up enclosing a number of smaller ones, 

 each exhibiting the concentric arrangement of cracks. 



This remarkable rock must be classed as di porphyritic and perlitic 

 mica-dacite r/lass. I ma}' add that while I have met with a number 

 of examples of the stony varieties of dacite in the district described in 

 this paper, this is the first occasion on which I have found it 

 assuming the perfectly vitreous character. It appears that this 

 vitreous variety of the rock occurs in scattered nests in the midst of 

 the ordinary stony form. I have seen, in the lava-streams of Lipari, 

 similar angular masses of glass enclosed in the stony rhyolites, and 

 the appearances in both the ancient and the recent rocks suggest 

 that a brittle glassy rock had been broken up and entangled in a 

 more slowly cooling mass that had assumed a stony character. 

 Subsequently this lava itself appears to have been broken up by a 

 volcanic vent being opened below it, and its fragments thus beoame 

 enclosed among the ejecta of the later volcano. 



There is still another point of great interest in connection with 

 this rock. Portions of it carefully dried at 110° C. and then weighed 

 were found, on ignition, to lose no less than 8-90 per cent, of their 

 weight. AVhen fragments of this glass were heated in a flame urged 

 by a powerful blast, they swelled up in cauliflower-like excrescences, 

 till they attained a bulk at least eight or ten times that of the original 

 fragment. The resulting product was found to be a beautiful white 

 pumice, which floats upon water. 



I have recently called attention to the same character as displayed 

 by the curious material known as Marekanite *. I find too that the 

 obsidian of Krakatao, which is a porphyritic enstatitt-dacite glass, 

 not very dissimilar in chemical composition to the rock we are 

 (considering, behaves in just .the same manner, when strongly heated, 

 and yields a dirty-white pumice, almost undistinguishable from the 

 natural pumice which was so copiously ejected from that volcano 

 during the great eruption of August 1883. 



The rocks of Angus and Fife are especially interesting to geologists 

 from the important light which they throw on the mode of 

 decomposition of some very interesting types of volcanic products. 



In the year 1874, I described the great masses of altered lavas 

 which had been ejected from the Old-Ked-Sandstone volcanos, under 

 the names of felstone, porphyrite, and claystone t- In the following 

 year I had the opportunity of examining the great andesitic vol- 



* Geo! Mag. dec. iii. vol. iii. (1886) p. 243. 



t Quart. Jourii. Geol. See. vol. xxx. (1874) pp. 277-289. 



