414 S. ALLPORT ON THE METAMOEPHIC ROCKS 



accurate measurements, I find myself compelled to differ from the 

 views maintained by Mr. Sorby in his well-known and valuable 

 paper on the " Microscopic Structure of Crystals," and which have 

 been adopted by Mr. Ward in his memoir on the granitic rocks of 

 the Lake-district, recently communicated to this Society*. 



A reference to Mr. Sorby's paper will show that his calculations 

 are founded on the following assumptions : — 



1. That the cavities were exactly filled with the fluid at the time 

 the crystals were formed. 



2. That the vacuity was produced solely by contraction on cool- 

 ing; and 



3. That the quartz of the various igneous rocks crystallized at 

 about 360° C, a temperature assumed to be that at which the tra- 

 chyte of Ponza might probably have been formed under a pressure 

 of about 4000 feet of rock. 



If the first two assumptions were true, it would of course follow 

 that the relative size of the cavity and bubble ought to be the same 

 in all quartz crystals found in any small portion of a rock, as, 

 according to the hypothesis, they must necessarily have been formed 

 at the same time and under the same conditions. This, however, is 

 so far from being the case, that it is difficult to find any speci- 

 men in which such uniformity exists. This difficulty appears, 

 in fact, to have presented itself to Mr. Sorby, who admits (p. 487) 

 that " there is sometimes a passage from fluid- to vapour- cavities 

 as if there had been an alternation of liquid and vapour or gases ; 

 both of which circumstances would be likely to occur ; " and again, 

 when describing certain vapour-cavities in the quartz of granite, he 

 says (p. 488) that they " gradually interfere with and pass into fluid- 

 cavities." 



Now the extremely probable occurrence of an intermixture of 

 fluid and gases ought surely to be regarded as sufficient to invalidate 

 any argument founded on the presence of a fluid only. It may afford 

 a satisfactory explanation of the great irregularity which certainly 

 does exist in the relative dimensions of the bubble to the liquid ; but, 

 as it is quite impossible to decide in what particular cases vapour of 

 some kind may have been enclosed together with the fluid, it is 

 difficult to see the value of any calculations based on such data. 



An escape from the difficulty, however, is suggested by Mr. Sorby 

 who says (p. 473): — "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 have happened with large cavities than with small. 

 There is also a greater risk of the large coming across flaws in the 

 crystal, so as to lose fluid. The very minute should, however, be 

 avoided. It is therefore better to select those of moderate size, 

 which Jiave vacuities of very uniform relative magnitude in parts 

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

 solid." Now, as to the directions here given, I would observe that 

 it certainly would be better to avoid such cavities as have caught 

 * Quart. Journ. Geol. Soc. toI. xxxi. p. 571. 



