LITHOLOGY. 1 83 



fluid expands under the simple pressure of its own vapor to the volume 

 of the cavity, while it is certain that the rock at the time it was made 

 must have been under a heavy pressure. Hence the relative volume of 

 bubble and cavity is dependent on two factors, one of which is the tem- 

 perature, and the other is the pressure at the time the rock was solidified; 

 and if the coefficient of expansion of the fluid is known, together with 

 either one of these factors, the other can be calculated. Such calcula- 

 tions were made by Mr. Sorby, and after him by Mr. J. Clifton Ward,* 

 according to the formula deduced by Mr. Sorby. Such calculations are 

 rendered of doubtful value by the large number of inconstants that are 

 introduced. The accuracy of the measurements on such minute and 

 irregular cavities is always doubtful, and neither one of the two factors 

 is known. Still, the observations possess weight when the results are 

 accepted as approximations, for, taking as one factor the lowest tempera- 

 ture which would suffice to render these materials plastic with the aid of 

 imprisoned water, the other factor is found to be very great; and hence 

 these rocks must have been formed and solidified deep down in the 

 bowels of the earth. This much can with certainty be assumed as the 

 teaching of these minute cavities. Great variations in relative size of 

 fluid and space, and in the expansibility of the fluid, caused by variable 

 amounts of carbonic acid, which is either free or held in solution, are 

 often observed in different cavities in the same crystal. This clearly 

 indicates that changes of condition took place even in the growth of a 

 single crystal. This variation in condition is also indicated by certain 

 other microscopic properties of crystals in our porphyries, which I have 

 already pointed out. 



The little cube which is so commonly seen in these cavities is proba- 

 bly of chloride of sodium or potassium. On heightening the tempera- 

 ture, this cube will dissolve. Its existence depends on the same causes 

 as does that of the bubble. Water, when heated and under pressure, 

 possesses a very great solvent power. Hence, when the temperature is 

 diminished and the pressure removed, the fluid, which has dissolved alka- 

 line salts from the feldspathic materials, becomes supersaturated. (See 

 further remarks in reference to this subject under Granite.) 



The bubbles in the very minute cavities, when highly magnified, are 



* Quarterly Jour. Geol. Society, No. 124, p. 568. 



