332 J. ARTHUR PHILLIPS ON THE ROCKS OF 



quartz is penetrated by acicular crystals of tourmaline, and contains 

 numerous gas- and fluid -cavities. It appears to have a decidedly 

 felspathic base, in which the various crystalline components are por- 

 phyritically enclosed ; in this respect, therefore, it closely approxi- 

 mates to the quartz-porphyries. 



Upon the assumption that at the temperature at which the fluid- 

 cavities in natural crystals were formed they were full of liquid, and 

 that when examined at lower temperatures they will be found to 

 contain cavities produced by a corresponding contraction of the 

 liquid, Mr. H. C. Sorby, F.K.S., has endeavoured in his admirable 

 paper " On the Microscopical Structure of Crystals," &c.*, to cal- 

 culate, approximately, the conditions, with regard to heat and pres- 

 sure, under which the rocks containing them were produced. 



In order to ascertain the relative size of the cavity and bubble, Mr. 

 Sorby measured, when practicable, long and regular cavities ; and he 

 states that this may also be determined, with a sufficient degree 

 of accuracy, from those which are equally deep throughout, and 

 which have a flattened vacuity, so that the proportion between the 

 areas is that between the volumes. " If, however, neither of these 

 can be found, the best approximation that can be made is to be de- 

 rived from such cavities as are nearly equiaxed, so that the relative 

 magnitude of the cavity and vacuity equals the cube of the ratio of 

 their diameters." He further remarks that ■'" in determining the 

 relative size of the vacuities in fluid- cavities, of course care must 

 be taken not to make use of such as have caught up bubbles of gas 

 along with the fluid, which is more likely to happen with large than 

 with small." "It is therefore best to select those of moderate size, 

 which have vacuities of very uniform relative magnitude, in parts 

 where vapour- or gas-cavities do not occur and the crystal is very 

 solid." 



This, however, is manifestly equivalent to choosing for the pur- 

 poses of experiment only those cavities in which the ratio of the 

 vacuity is as nearly as possible constant, and which must of ne- 

 cessity yield concordant results. When, therefore, we also con- 

 sider the difficulty of accurately measuring cavities of which the 

 longer diameter varies from -^ T) to yxrW °f an inch? an <l further 

 take into account the impossibility of finding cavities of which 

 the depth can be proved to T^e uniformly equal, it will be ad- 

 mitted that this method, by measurement, is open to grave sources 

 of error. 



In order, if possible, to avoid these objections, I have attempted to 

 determine, by direct observation, the temperature at which the 

 bubbles in Cornish granites, elvans, and veinstones, disappear. For 

 this purpose a microscope, supported horizontally, and provided with 

 both coarse and fine adjustments, was employed. A rectangular bath 

 of stout sheet brass was furnished on the side nearest the object- 

 glass with a circle of thin covering-glass, secured in its place by 

 paper washers and a screw ring. On the opposite side, a thicker 

 glass was ground into a groove in the metal, and fastened by a 

 * Quart. Journ. Geol. Soc. Nov. 1858, vol. xiv. pp. 453-500. 



