26 



the bubbles observed in the fluid inclusions of the minerals in rocks are 

 produced in the same way, then the ratio above referred to must also be 

 constant, and must bear some relation to the temperature and pressure at 

 which the minerals were formed. 



The following equation, based on numerous experiments, expresses this 

 relation according to Dr. SORBY. 



v =(Bt + Ct 2 ) (1 -00000271 p) -00000271 p. 



Where v = volume of bubble estimated in terms of the liquid which is 



taken as unity. 



t = temperature at which the mineral was formed. 

 p = pressure over and above that which would be naturally due to 



water vapour at a temperature t. 



B and C are constants which depend on the nature of the liquid, and 

 which have been determined experimentally by Dr. SORBY for 

 water and certain saline solutions, such as those which occur in 

 liquid inclusions. 



In this equation we have two variable quantities, only one of which (u) 

 is known in the case of the minerals occurring as rock constituents. The 

 temperature can only be ascertained when the pressure is known, or the 

 pressure when the temperature is known. Assuming that granites are formed 

 at a temperature of 360 C. for reasons given in the paper, but which need 

 not here be considered, DR. SORBY has endeavoured to determine the pressure 

 under which different varieties have been formed. 



Mr. CLIFTON WARD applied the same method to the granites of the 

 Lake District^ 1 ' 



It is much to be regretted that more extended observations have not 

 tended to establish the general applicability of this method, or to give 

 confidence in the accuracy of results obtained by means of it. It is based on 

 the assumption that water and water- vapour are the only fluids present in the 

 inclusions an assumption which in many cases is probably incorrect. The 

 experiments of VOGELSANG and GEISSLER for instance proved the existence of 

 carbon dioxide in granite of Johann Geordenstadt. Again, the observations of 

 Mr. PmLLiPS (2) and others tend to show that the constancy in the ratio of the 

 volume of the bubble to that of the liquid, which is absolutely essential to the 

 applicability of the method, does not exist ; and that every gradation may be 

 found in the same rock, or even in the same mineral, between inclusions which 

 are nearly or quite full of liquid, and others which are empty or occupied 

 only by a gas. 



" Messrs. EENARD and HE LA VALLEE PoussiN v8) have devised a method for 

 determining the temperature at which certain rocks have been formed 

 depending upon the solubility of salt in water. Liquid inclusions containing 

 cubic crystals of common salt occur in the diorite of Quenast, Belgium. In 

 one of these the volume of the liquid was estimated at '0000002198687 

 cubic mm., and that of the salt at '0000000098003 cubic mm. REGNAULT 

 ascertained by experiment that the solubility of sea salt in water increases in 

 direct proportion to the temperature between 0C. and 120C. and that atOC. 



(T)~Q. J. a. S., vol. XXXI., 1875, p. 388. 



(2) On the Rocks of the Mining Districts of Cornwall. Q. J. G. S., Vol. XXXI., p. 332. 

 See also ALLPORT. Q J. G. S., Vol. XXXII , p. 413. 



(3) Memoire sur les Roches dites Plutoniiieiines. Brussels, 1874. 



