On the Fissure-Inclusions. 105 



narrow rim, and so differing from the well-known appearance of 

 air bubbles in balsam or other liquids. (2) Cavities completely 

 filled by a colorless liquid, or by more than one, of different 

 refractive powers, however, than the enveloping matrix, and 

 with sharply defined outline. (3) Cavities only partially 

 occupied by a liquid which is associated with a bubble of gas; 

 and (4) cavities of either of the preceding varieties enclosing 

 crystals or particles of some solid substance {e. g. common salt, 

 needles of hornblende, or tourmaline, etc.). The cavities 

 themselves are always very minute, rarely exceeding 0.06 mm. 

 in diameter. 



They may be occasionally distinguished, at least in groups, 

 under very low powers, but the smaller ones (in which alone 

 the bubbles with spontaneous motion are found) can only 

 rarely be identified under a y*^ inch objective, or, when pre- 

 viously selected, sometimes under a -/z inch ; ordinarih' a ^ or 

 y^ inch objective is required for satisfactory definition. Their 

 forms vary widely, the smaller being generally rounded — 

 spherical, elliptical, oval, etc. — or larger and coalescing, and 

 then far more irregular, or even angular, presenting crystalline 

 forms. 



The fluids which occupy them are confined to a small class, 

 the liquid being generally water, or some saline solution, some- 

 times a bituminous liquid, and sometimes liquid carbonic acid 

 or its solution in water. In the rocks of the Eastern U. S.. 

 this last interesting substance may be of rare occurrence, 

 having been thus far reported only in two syenytes from New 

 Hampshire,* but it occurs abundantly in the granites of the 

 western territories.! The gas may be air, watery vapor, car- 

 bonic acid, or other gas. 



The size of the bubble varies greatly, sometimes predomi- 

 nating over that of the liquid, though generally amounting to 

 not over J^ of its volume. When large, the bubbles may cling 

 to one or opposite walls of a cavity, or may be compressed and 

 flattened between them, and are generalh' fixed ; but even 

 these often change their place on heating, or rotation of the 

 stage, following the apparent lower side of the cavity. How- 

 ever, they often show a slow and continuous rocking or pendu- 

 lous motion from side to side, as if fixed on a pivot. This 



*Geol. of N. H., Vol. III., Part IV., 207. G. W. Hawes. 



+U. S. Geol. Expl. of the 40th Parallel, VI., 20, 33, 44. F. Zirkel. 



