1074 THE MICKOSCOPE IN GEOLOGICAL INVESTIGATION 



under great pressure by the combined influence of liquid water and 

 fused mineral matter the crystals will contain glass-cavities and also 

 fluid inclusions. 



Glass-inclusions are very abundant in the porphyritic crystals 

 of volcanic rocks, and represent to some extent the composition of 

 the fused mass at the period of inclosure. The glass composing the 

 inclusions is often darker in colour than the glass forming the base 

 of the rock. This is perhaps due to the presence in the glass of 

 the inclusions of a greater amount of iron and the bases usually 

 associated with it. The glass often contains crystallites and micro- 

 lites, due sometimes to inclosure at the same time, sometimes to a 

 subsequent crystallising action set up by the glass. Gas bubbles 

 are also inclosed. 



The existence of fluid inclusions in crystals has long been kno\vn ; 

 but not until Dr. Sorby directed his attention to the subject was 

 their universal distribution in rock- constituents imagined, or their 

 bearing upon geological problems recognised. They are often very 

 minute, being frequently less than ro^oijth of an inch in diameter. 

 They are rare or absent in rocks of the volcanic group, but are 

 especially characteristic of the plutonic rocks, such as granite, 

 gabbro, diorite, &c. Where glass-inclusions are common, fluid 

 inclusions are rare or wanting. 



The forms of such inclusions vary, but sometimes they are 

 bounded by planes corresponding to the external faces of the crystals, 

 in which case they are termed ' negative ' crystals. 



Sometimes the fluid inclusions are so numerous in the quartzes 

 of the granites as to be, according to Dr. Sorby, 1 ' not above the 

 j^yoth of an inch apart. This agrees with the proportion of a 

 thousand millions to a cubic inch, and in some cases they must U- 

 more than ten times as many.' 



An intimate relation usually exists between the number of 

 cavities in a crystal and the rate at which it was formed. 

 Generally speaking, it may be said that the more rapid the growth, 

 the more numerous the inclusions. 



Not infrequently the cavities contain bubbles varying from 

 r^iRFoth *o s^jy^oth f an i ncn i n size. These bubbles sometimes 

 possess an apparently spontaneous movement, at other times heat 

 must be applied to produce a change of position. 



According to Dr. Sorby's experiments, the bubbles arise in con- 

 sequence of the contraction of the liquid on cooling from the high 

 temperature at which the cavities were filled. 



The nature of the inclosed fluid has been determined with some 

 accuracy. Generally the liquid is a solution of water charged with 

 salts ; but occasionally it is sufficiently concentrated to cause the de- 

 position in the cavities of little cubes of salt. The presence has also 

 been established of liquid carbonic dioxide, the bubble of which dis- 

 appeared at about 32 C., the critical point for this gas. 2 



The discovery in the mineral components of plutonic rocks of 



1 Sorby, Quart. Journ. Geol.- Soc. 1858, p. 486. 



2 The application of the burning end of a cigar to the section is usually sufficient 

 to cause the bubble to disappear. 



