

\ 
96 ---GEOGNOSY. <-> Bagman 
* 7} 
> +. «ire 
a, Gas-filled or empty cavities—are most frequently — 
globular or elliptical, and appear to be due to the presence of 
gas or steam in the crystal at the time of consolidation. Zirkel 
estimates them at 360,000,000 in a cubic millimetre of the hauyne 
from Melfi.1 In some instances the cavity has a geometric form 
belonging to the crystalline system of the enclosing mineral. Such 
a space defined by crystallographic contours is a negative crystal. A 
cavity filled with gas contains no bubble, and its margin is marked by 
a broad dark band. The usual gas is nitrogen, with traces of oxygen 
and carbon dioxide; sometimes it is entirely carbon dioxide or 
hydrogen and hydrocarbons. — 
B. Vesicles containing liquid (and gas.)—As far back as 
the year 1823, Brewster studied the nature of certain fluid-bearing 
cavities in different minerals.” The first observer who showed their 
important bearing on geological researches into the origin of 
crystalline rocks was Mr. Sorby, in whose paper, already cited, they 
occupy a prominent place. Vesicles entirely filled with liquid are 
distinguished by their sharply-defined and narrow black borders. 
Vesicular spaces containing fluid may be noticed in many artficial 
crystals formed from aqueous solutions (crystals of common salt show 
them well) and in many minerals of crystalline rocks. ‘They are ex- 
ceedingly various in form, being branching, curved, oval, or spherical, 
and sometimes assuming as negative crystals a geometric form, like 
that characteristic of the mineral in which they occur, as cubic in 
rock salt and hexagonal in quartz. They also vary greatly in size. ~ 
Occasionally in quartz, sapphire and other minerals large cavities are 
readily observable with the naked eye. But they may be traced with 
high magnifying powers down to less than ;,4,,5 of an inch in 
diameter. Their proportion in any one crystal ranges within such 
wide limits, that whereas in some crystals of quartz few may be ob- 
served, in others they are so minute and abundant that many millions 
must be contained inacubicinch. The fluid present is usually water, 
frequently with saline solutions, particularly chloride of sodium or of 
potash, or sulphates of potash, soda, or lime. Carbon dioxide may be 
present in the water; sometimes the cavities are partially occupied 
with it in liquid form, and the two fluids, as originally observed by 
3rewster, may be seen in the same cavity unmingled, the carbon di- 
oxide remaining as a freely moving globule within the carbonated 
water. Cubic crystals of chloride of sodium may be occasionally ob- 
served in the fluid, which must in such cases be a saturated solution of 
this salt (ig. 7, lowest figure in Column A). Usually each cavity con- 
tains a small globule or bubble, sometimes stationary, sometimes 
movable from one side or end of the cavity to the other as the specimen 
is turned, sometimes slowly pulsating from side to side, or rapidly 
vibrating like a living organism. ‘he cause of these movements 

1 Mik. Beschaff. p. 86. 
* Ldin. Phil. Journ. ix. p. 94. Trans. Roy. Soc. Edin. x. p.1. See also W. Nicol. Edin. 
New Phil. Journ. (1828) v. p. 94. 
