

'22 



sandstone, granite, &c., to the action of fused masses of igneous rocks for one 

 or two hours. 



A basalt-magma appears to be the one which is most capable of 

 developing secondary glass inclusions, and quartz is the mineral in which such 

 inclusions are most readily developed. According to M. CHBU8TSCHOFF they 

 may be formed in two ways : (1) by the passage of the magma into cracks 

 and cavities in the mineral, and the subsequent sealing up of the 

 channels of communication, and (2) by the fusion of enclosed minerals. 

 A basic magma will act chemically on quartz and take up a portion of 

 its substance until it becomes saturated with silica. In this way pre-existing 

 cracks and cavities may be enlarged. Many of the secondary inclusions have 

 the form of negative crystals, a fact which shows that in all probability the 

 solvent action is exerted unequally in different directions. The absence of 

 any channel connecting the inclusion with the exterior is explained, according 

 to M. CHRUSTSCHOFF, by the deposition of secondary silica from the magma 

 during the process of cooling. Such silica would necessarily be deposited in 

 optical continuity with the original crystal, and might therefore obliterate the 

 connecting channel. In certain cases there is direct evidence of the deposition 

 of such secondary silica. When glass inclusions arise in consequence of the 

 fusion of minerals contained in the original crystal, the absence of a 

 connecting channel requires of course no explanation. 



In studying glass inclusions in microscopic sections it must of course be 

 remembered that it is not always possible to determine whether a given mass 

 of glass which is entirely surrounded by crystal-substance is, in reality, an 

 inclusion. It may be that the section has been cut in such a direction as not 

 to expose the channel of communication. 



FLUID INCLUSIONS. The existence in certain crystals of cavities con- 

 taining liquids and gases has long been known. In 1822 SIR HUMPHREY 

 DAVY (1) proved that the liquid in a large cavity, in a certain quartz crystal, 

 was water ; and that the bubble which could be seen to move about like that 

 of a spirit level, always keeping to the uppermost side of the cavity, as the 

 position of the crystal was changed, was either nitrogen or a vacuum. 

 SIR DAVID BREWSTER (2) recognised the existence of fluid inclusions in a large 

 number of minerals and discussed the bearing of these inclusions on questions 

 relating to the origin of the minerals in which they occur. He also detected 

 the existence of two immiscible liquids in certain cavities in topaz and other 

 minerals, and observed that one of these liquids was twenty times more 

 expansible with increase of temperature than water. 



It was not, however, until Dr. SORBY turned his attention to the subject, 

 and published his classic paper on " The microscopical structure of crystals 

 indicating the origin of minerals and rocks," that fluid inclusions were known 

 to be widely distributed in the ordinary rock-forming constituents, and to have 

 an important bearing on geological problems. 



The fluid inclusions which occur in the constituents of rock-masses are 



(1) Phil. Trans. Roy. Soc., 1822, p. 367. 



(2) Trans. Roy. Soc. Edin. Vol. X., pt. I, p. 1. 1824. 



