306 J. W. Judd — On Volcanos. 



ocean, or possibly that of mountain masses of volcanic materials 

 poured out at the surface and piled above them, has evidently influ- 

 enced their mode of consolidation, and greatly modified their cha- 

 racters. 



This conclusion is quite in accordance with the microscopical 

 characters presented by the minerals which compose these rocks. 

 The felspar crystals abound with cavities filled with stony matter; 

 while the crystals of quartz, as pointed out by Mr. Sorby, contain 

 fluid-cavities with air-bubbles, and present a most perfect resem- 

 blance to those of the true granitic rocks. The result of Mr. 

 Sorby's most ingenious researches, however, was to show that while 

 the quartz crystals of the Ponza rock must have been formed under 

 a very considerable pressure (one of possibly not less than 4000 

 feet of rock), yet that the ordinary granites were produced under 

 a pressure which must have been far greater. 



Great, indeed, as are the points of resemblance between the 

 rock of Ponza and many granites, both in chemical and mineralogical 

 constitution, and in certain features of their microscopic structure, 

 the real and important points of difference between these two classes 

 of rock must not be lost sight of. These differences consist in the 

 tendency which the basis of the rock constantly shows to assume the 

 vitreous condition, and in the mode of arrangement and injured con- 

 dition of its embedded crystals. In these respects the rock of Ponza 

 approaches and even graduates into the ordinary highly siliceoxis 

 lavas (Quartz-trachytes, Liparites or Bhyolites), such as those which 

 we have described in the Lipari Islands. 



Thus we are led to the conclusion that rocks like those of Ponza, 

 and certain others in the Euganean Hills, Hungary, etc., which pre- 

 cisely agree with them in character, form a perfect bond of connexion 

 between the granites on the one hand and the highly siliceous lavas 

 (Liparites) on the other. For rocks of this character Pichthofen has 

 suggested the name of " granitic-rhyolite," or " Nevadite, " and his 

 definition of this rock, which constitutes great mountain masses in 

 the western parts of North America, appears to be entirely applic- 

 able to the rock of Ponza. "Whether geologists agree to accept this 

 term or not, the fact remains of the existence of a series of rocks 

 through which we can trace the passage, by the most insensible 

 gradations, from granite to the variety of lava known as Liparite. 



It has been shown by Delesse, Durocher, and other observers, that 

 a rock of highly crystalline or granitic structure has a much higher 

 specific gravity than the glass formed by its artificial fusion. As 

 both mathematical reasoning and experiment have led Sir "William 

 Thomson and his brother to the conclusion that for those bodies 

 which contract in consolidation pressure raises the point of fusion, 

 while for those that expand it lowers it, we might by analogy be 

 justified in inferring that, under great pressure, rocks would be 

 unable to undergo that expansion necessary for their assuming the 

 colloid or vitreous condition. I need not point out how this con- 

 clusion coincides with the observations of the geologist. "We have 

 the strongest grounds for inferring that in granite consolidation took 



