FORMATION OF THE VARIOUS INCLUSIONS 119 



fresh trap, and so a later decomposition can not have furnished the 

 calcite* 



FORMATION OF LITHOPHYS^ WITH SPH.EROCRYSTALS OF ANKERITE AND 



QUARTZ 



(See plate 28, figures 3, 4, and plate 29.) 



In the cases thus far considered there was no excess of water or gas 

 expelled in the formation of the glass, or the pressure prevented its 

 expansion. 



In other cases the formation of the glass was accompanied by such 

 expulsion, and the water or gas expanding forms a cavity, after the 

 manner of lithophysse on a very small scale, in the midst of the glass, 

 and thus probably further increased the amount of the glass by the ab- 

 straction of heat. In these highly silicious patches, where silica and 

 calcite have later crystallized out abundantly among the common con- 

 stituents of the basic rock, conditions may have suddenly supervened 

 (as, for example, a lower temperature) in the liquid magma which would 

 permit the Si0 2 to expel C0 2 from the carbonate. Against the weight of 

 100 feet of the magma and its viscosity the gas or water vapor ex- 

 panded to form cavities, sometimes a half inch across, and the walls of 

 these cavities congealed on the instant (from* the heat abstracted in the 

 expansion) into filmy spheres of glass like soap bubbles, and these (ex- 

 panded by the explosion beyond a size they could maintain) shriveled, 

 their plastic walls crumpling into complex wrinkles or buckling into the 

 interior in sharp folds or, where more rigid, cracking and sliding past 

 each other. In other cases the collapse was entire, and the shards of 

 glass are seen by the microscope enveloped in the sphserocrystals of cal- 

 cite or quartz which follow or scattered abundantly in the surrounding 

 crystalline ground (see the middle of the sphserocrystal in plate 29). 

 When the wall was more plastic it collapsed partly without fracture, and 

 the continued development of the glass on the outside in the midst of 

 the crystallizing magma gave a minute and perfect botryoidal inner 

 surface to the glass cavities. 



One can see that these cavities were instantly occupied by solvent 

 fluids — superheated steam, perhaps — because every point of the perfectly 

 fresh fractured or bent glass projecting into the cavity bristles with radiate 

 tufts of calcite or ankerite rhombs, while some of these tufts increase into 

 the large perfect banded and radiate ankerite sphserocrystals which pro- 



* The highly ferruginous glass produced by the refusion of the pyroxene (see page 100) is ex- 

 cluded from this discussion. It is a deep red brown ; this is yellow. It solidified in the imme- 

 diate vicinity of the pyroxene without dissolving any other mineral ; this is a residuum of the 

 magma which abnormally held calcite and quartz in solution. 



