I04 Avierica7i Quarterly Microscopical Journal. 



by fluid cavities, as well as of certain conclusions accepted by 

 lithological authorities, which seem to require some modifica- 

 tion in view of the facts presented by a study of this gneiss. 



The term " inclusion " is applied by micro-lithologists to the 

 minute and merely accessory bodies which are enclosed in the 

 mass of an otherwise pure, cr}'stallized mineral or homogeneous 

 rock, and which seem, in most cases, to have been caught with- 

 in its material during the process of crystallization. 



Macroscopically (/'. e. in a form sufficiently large to be visible 

 without the aid of the microscope), many of these bodies are 

 very familiar to every mineralogist as granules, films, and tiny 

 cr}-stals {e.g. the magnetite and ochre in the interstices of horn- 

 blende, the acicular needles of rutile in quartz, the scales of 

 specular iron in aventurine feld-spar, etc.,) and as large cavities 

 {e.g. those holding water in the cry^stals of quartz, calcite, etc.); 

 to ever}^ geologist in the globules and veins of glass or obsidian 

 enclosed in many lavas; and to every chemist in the mother- 

 liquor caught up during the crystallization of salts. But it is only 

 beneath the microscope that the extreme abundance of these 

 bodies in all crystallized substances can be recognized, and their 

 identity, characteristics, and significance satisfactorily deter- 

 mined. 



In thin sections of all minerals sufficiently translucent to 

 admit of this kind of investigation, whether in isolated crystals 

 or in those irregular and generally crystalline aggregates that 

 we call rocks, these inclusions are constantly found, and ac- 

 count in part for the perplexing inaccuracy and variation of 

 many skillfully made chemical anal3'ses, and in part for certain 

 physical characteristics, such as decrepitation, opacity, color, 

 variations in specific gravity, etc. 



The one class which will be discussed in this paper, fluid-in- 

 clusions, often abound in gems, such as beryl, topaz, sapphire* 

 ruby, etc., and even affect their translucency; and the charm of 

 their study in this delightful material, by such investigators as 

 David Brewster, Sir Humphrey Davy, H. C. Sorby, and in this 

 country Isaac Lea, has exerted considerable influence on the 

 careful attention which has since been given to their occurrence 

 in all mineral forms, by investigators like Vogelsang, Zirkel, 

 Rosenbusch, and others. Four common varieties of these in- 

 clusions may be often seen, sometimes in the same field of view: 

 (i) Cavities enclosing air, or a gas, distinguished by a dark but 



