338 J. ARTHUR PHILLIPS ON THE ROCKS OF 



In fig. 2, which represents another crystal from the same locality, 

 magnified to a similar extent as the foregoing, several separate 

 intrusions appear to have coalesced, while the openings by which 

 they were admitted have again closed ; in addition to the larger mass 

 this specimen encloses several others, together with numerous gas- 

 and fluid-cavities. 



Fig. 3. This specimen is from the same elvan, and is represented 

 as magnified to the same extent as the before-described sections ; 

 it exhibits a considerable number of spherical particles of base upon 

 which the substance of the crystal has subsequently closed. These 

 included globules of base are sometimes hollow, and are then merely 

 gas- or vapour-cavities internally lined with a thin coating of basic 

 material. The nature of these bodies may sometimes be determined 

 by the repeated examination under the microscope of a section during 

 the process of preparation ; and should any cavity be present, it 

 will often be observed to be laid open. 



In fig. 4, which is a quartz crystal (magnified to the usual extent) 

 from the Trerice elvan, all the detached portions of base have, under 

 the influence of forces causing the crystallization of the quartz, as- 

 sumed a quasi-crystalline form. 



Prom the nature of these intrusions into the quartz of elvans, or 

 quartz -porphyries, it would appear that the crystals, while in a plastic 

 state, and previously to assuming their final configuration, experi- 

 enced an irregular contraction, giving rise to channels by which a 

 portion of the still liquid or pasty base was drawn into their sub- 

 stance. In some cases the force of crystallization, subsequently 

 exercised, would seem to have moulded them into crystalline forms, 

 while in others it has apparently not been sufficiently powerful to 

 effect this result. 



It has been observed by Mr. Sorby, who accounts for them by 

 supposing a portion of the base to have been " caught up" in the 

 crystal, that " stone-cavities are not well developed except in granites 

 whose structure approximates somewhat to that of elvans "*. A 

 careful examination, however, of the quartz of elvans tends to throw 

 considerable light on the mode of formation both of stone- and 

 glass-cavities. The crystals containing them have, by irregular 

 contraction while in a plastic state, drawn into their substance 

 portions of the surrounding base ; in granites and quartz- porphyries 

 having a stony base the result will evidently be stone -cavities, 

 while in pitchstones and other similar rocks glass- cavities will be 

 produced. 



The ultimate chemical and mineralogical composition of granites 

 and elvans being the same, they differ only in the mode of aggrega- 

 tion of their constituents ; and they may therefore be assumed to 

 have been derived from a common source, but to have become con- 

 solidated under different conditions. 



On referring to the Table, p. 322, showing the composition of 

 various Cornish clay-slates, it becomes evident that neither gra- 

 nites nor elvans could result from the rearrangement, by heat or 

 * On the Microscopical Structure of Crystals. &c. 



