

24 



b. 

 Fig. 11. Fluid Inclusions (After Sorby). 



(a) Fluid inclusion in quartz of Aberdeen granite. Magnified 2000 diameters. 



(6) Group of fluid inclusions in quartz of granite at St. Austel, Cornwall. Magnified 200 

 diameters. 



(c~) Fluid inclusion, in the form of negative crystal, containing a bubble and a crystal of salt, 

 in quartz of granite from Ding Dong Mine, Cornwall. Magnified 2000 diameters. 



The number of inclusions is also liable to great variation. Sometimes 

 they are scattered sparsely through the crystals, at other times they are 

 collected together in such numbers as to give them a cloudy aspect. Dr. SORBY 

 says (1) : " In many granites the fluid cavities are so numerous in the quartz, 

 that on the average they are not above the r^W of an inch apart. This agrees 

 with the proportion of a thousand millions to a cubic inch, and in some cases 

 they must be more than ten times as many." 



Under the microscope it is possible to recognise, in the larger cavities, 

 the existence of one and more rarely of the two liquids and a bubble. In the 

 larger cavities the bubbles may usually be caused to move, like those of spirit 

 levels, by placing the microscope in a horizontal position and rotating the 

 stage. In the smaller cavities, which contain bubbles varying from T-S&OTT to 

 6-0 hnr of a inch, the bubbles frequently change their positions without any 

 variation in the position of the slide. Sometimes this movement, which is 

 often termed the Brownian movement, is merely a kind of vibration ; at other 

 times the bubble wanders about all over the cavity in an apparently 

 spontaneous manner. The most minute bubbles show the most rapid 

 movement.' 2 ' It is, however, of great utility to the petrographer as 

 furnishing indisputable evidence of the fluid nature of the inclusions in 

 which it occurs. In addition to the bubble, cubical and sometimes also 

 prismatic crystals may occasionally be observed in these inclusions. 



Great care has been exercised by SORBY, VOGELSANG, and PFAFF in 

 determining the nature of the contents of the fluid inclusions. SORBY expelled 

 the fluid contents by heating small fragments in a long glass tube, one portion 

 of which was kept cold by a mixture of ice and salt. He observed that a 

 solid substance was condensed on the cold part of the tube which possessed 

 the crystalline properties and thawing point of ice. He also washed the 

 powdered material with pure water and tested the solution thus formed with 

 various reagents. In this way he established the existence of the chlorides of 

 sodium and potassium, the sulphates of sodium, potassium and calcium, and 

 sometimes also of free acids which may, however, have been produced by 

 the partial decomposition of the salts by the heat necessary to expel the liquids 

 from the inclusions. The general conclusion of SORBY is that the liquid 

 present in the cavities of the constituents of plutonic rocks is usually water 

 charged with the above-mentioned salts in solution. 



(1) Op. cit., p. 486. 



(2) For a discussion as to the cause of this movement see HAETLEY, Proc. Roy. Soc., vol. 



XXVI., pp. 137 and 180. 



