
98 GEOGNOSY. _: + gNBote Te 
provisionally, though other considerations go to indicate that it is 
probably not far from the truth. Assuming then that this was the 
temperature at which the vesicle was formed, these authors proceed 
to determine the pressure necessary to prevent the complete vapori- 
sation of the water at that temperature, and obtain as the result a 
ay 
“he 
» 

pressure of 87 atmospheres, equal to 84 tons per square foot of 
surface. The great pressure under which many rocks were formed is 
well shown by the liquid carbon dioxide in the pores of their crystals. 
Fluid inclusions may be dispersed at random through a crystal, 
or, as in the quartz of granite, gathered im intersecting planes 
(which look like fine fissures and which may sometimes have 
become real fissures owing to the line of weakness caused by the 
crowding of the cavities), or disposed regularly in reference to the 
contour of the crystal. In the last case they are sometimes confined to 
the centre, sometimes arranged in zones along the lines of growth of 
the crystal.? They are specially conspicuous in the quartz of granite 
and other massive rocks, as well as of gneiss and mica-schist; also 
in felspars, topaz, beryl, augite, nepheline, olivine, leucite and other 
minerals. 
y. Inclusions of glass or of some lithoid substance. 
—In many rocks which have consolidated from fusion, the component 
crystals contain globules or irregularly shaped enclosures of a 
vitreous nature (Fig. 7, Column B). These enclosures are analogous 
to the fluid-cavities just described. They are portions of the 
original glassy magma out of which the minerals of the rock crystal- 
lized, as portions of the mother-liquor are enclosed in artificially 
formed crystals of common salt. That magma is in reality a liquid 
at high temperatures, though at ordinary temperatures it becomes a 
solid. At first these glass vesicles may be confounded with the 
true liquid cavities which in some respects they closely resemble. 
But they may be distinguished by the immobility of their bubbles, 
of which several are sometimes present in the same cavity ; by the 
absence of any diminution of the bubbles when heat is applied ; by 
the elongated shape of many of the bubbles; by the occasional 
extrusion of a bubble almost beyond the walls of the vesicle, by the 
usual pale greenish or brownish tint of the substance filling the 
vesicle, and its identity with that forming the surrounding base or 
ground-mass in which the crystals are imbedded ; but above all, b 
the complete passivity of the substance in polarized light. (See 
§ vii., p. 188.) 
Glass inclusions occur abundantly in some minerals, aggregated 
in the centre of a crystal or ranged along its zones of growth with 
singular regularity. ‘They appear in felspars, quartz, leucite, and 
other crystalline ingredients of volcanic rocks, and of course prove 
* Mémoire sur les Roches dites Plutoniennes de la Belgique, De la Vallée Poussin 
A. Renard. Acad. Itoy. Belg. 1876, p. 41. See also Ward, 0. J. Geol. Soc. xxxi. p. nae 
2 fT . ‘ ; ic ragicla » . » . ‘ 
The way in which vesicle B, enclose d crystals, &c., are grouped along the zones of 
growth of crystals is illustrated in Fig. 5, 
