304 J. W. Judd — On Vokanos. 



3. These crystals, when embedded in the rock, are often seen to 

 be rounded on their edges and to have suffered other injuries. Some- 

 times the same crystal is seen broken into several fragments, which 

 are more or less separated from one another. The mica crystals, 

 owing to their perfect cleavage, have often especially suffered ; their 

 edges are " frayed-out," their laminae separated by portions of the 

 matrix which has been forced between them, and occasionally the 

 plates of which they are built up are found to be twisted and 

 crumpled in the most extraordinary manner. 



4. Such crystals are all seen to be arranged with their longer axes 

 in the direction of the flow, and around them the smaller crystals, 

 formed by the devitrification of the enveloping mass, exhibit the 

 fluidal structure and a peculiar packing or condensation around and 

 behind them. Many of the sections indeed present an appearance 

 which may be justly compared to the surface of a flowing stream, on 

 which at the same time quantities of chaff and a number of pieces 

 of wood are floating; the former representing the microliths, and the 

 latter the porphyritically embedded crystals. 



That those conditions of high temperature, great pressure, and the 

 presence of large quantities of imprisoned water and gases, which 

 exist deep down in a volcano, are eminently favourable for the 

 formation of large crystals of various minerals, we have the clearest 

 proof in the beautiful contents of those blocks which are torn from 

 the deep underlying rocks of Vesuvius and ejected from its throat. 

 That the same conditions should induce a similar separation of the 

 materials of the liquefied mass itself, is no more than might be 

 expected. On a future occasion I shall discuss the nature and 

 origin of the condition of fluidity in igneous rocks, upon which so 

 much light is thrown by the fact that crystals of minerals of very 

 different degrees of fusibility are able, not only to separate, but to 

 continue floating about in them. 



II. When, as was shown by Mr. Sorby, a granite, like that of 

 Mount Sorrel, is fused, it passes on cooling into a glass. But if the 

 cooling be conducted slowly, sphcerulites composed of acicular crystals 

 in radial groups are formed in the mass. Now in some cases the 

 matrix surrounding the crystals of the Ponza rock before described 

 has assumed a vitreous condition, and it there becomes a porphyritic 

 obsidian. In this obsidian every variation from the first ap- 

 pearance of crystalline structure to the formation of the most distinct 

 sphserulites may often be observed. 



III. If glass be heated to a point far short of that required for its 

 fusion and slowly cooled, crystals of various minerals begin to make 

 their appearance in the mass, which gradually passes into stone, or 

 in other words becomes devitrified. The possibility of this passage 

 from the glassy to the stony condition without fusion is a condition 

 which must always be borne in mind by the geologist. The slow- 

 ness with which large masses of such imperfectly conducting ma- 

 terials, as most lavas are, cool down, is familiar to all who have 

 studied volcanos. It can hardly fail to happen, then, that maiiy 

 lavas which have solidified as glasses have, in the long intervals, 



