J. W. Judd — On Volcanos. 305 



during which they have been gradually parting with their remaining 

 heat, become devitrified. 



The glassy condition of rocks is clearly an exceptional and unstable 

 condition for them to assume. The probable reason why no vitreous 

 rocks of ancient date exist is not because similar conditions of 

 volcanic action did not prevail in earlier periods of the world's his- 

 tory, but because the vitreous rocks have lost their peculiar charac- 

 ters by devitrification. In proof of this conclusion I may recal the 

 fact, already described by me, of Old Eed Sandstone lavas in Scot- 

 land exhibiting traces of sphaerulitic structure, which appears to be in 

 all cases connected with the existence of volcanic glass. Even a 

 moderate degree of heat, if sufficiently prolonged, permits of the 

 passage of a matter from the unstable colloid to the stable crystalline 

 condition ; and it is not improbable that pressure and other forces 

 long sustained may be attended with the same result. 



The Ponza rock often exhibits clear evidence that after solidifying 

 in the form of a glass it has been subjected to devitrification. 



IV. The passage through the rock of water, especially when this 

 contains such acids as abound in volcanic regions, may completely 

 alter the composition and internal characters of the rock. Certain 

 minerals among .its constituents may be attacked and removed in 

 solution, while others assume a totally different crystalline condition 

 and arrangement. Of such changes the rock of Ponza often exhibits 

 the clearest evidence, its more basic materials being attacked and 

 destroyed, and its quartz re-crystallized. As shown by Mr. Scrope, 

 veins of quartz and true metallic lodes with cupriferous pyrites occur 

 in this rock ; and the quartz of these, as pointed out by Mr. Sorby, 

 is quite different in character from that in the unaltered igneous 

 rock. It contains "many fluid-cavities with water holding in solution 

 the chlorides of potassium and sodium, the sulphates of potash, soda 

 and lime, and free hydrochloric acid." 



Let us now proceed to inquire what are the relations of this 

 interesting rock of Ponza to granite, on the one hand, and to the 

 ordinary highly siliceous lavas (quartz-trachytes or Liparites), on the 

 other. 



The geological relations of this rock have been so fully illustrated 

 by Mr. Scrope that it is not necessary to dwell at any length upon 

 the subject. Through vast masses of pumiceous agglomerates, evi- 

 dently formed by explosive action, the solid rock of which we are 

 speaking has been forced in dykes and sheets, which sometimes have 

 a width of a few inches only, at others of many yards. The crushed 

 and re-consolidated character of portions of the matter at the sides of 

 these dykes, the remarkable banded and ribboned internal structure 

 of the rock itself in many places, and the phenomena witnessed at 

 the planes of contact of the dykes with the masses which they tra- 

 verse, bear witness to the violent force and vast irregular pressures 

 which accompanied their intrusion. 



In no case does this more ancient rock of Ponza appear to have 

 been extruded as lava, and to have consolidated under ordinary at- 

 mospheric pressure. Either the pressure -of the superincumbent 



DECADE II. — TOL. II. NO. VII. 20 



