222 J. D. Dana — History of the Changes in Kilauea. 



figures, a few of which are here copied, the glassy fibers are 

 sometimes forked or branching ; sometimes welded at crossings ; 

 often contain air-vesicles (3, 4), and microscopic crystals (1, 2, 

 5) ; often tubular (1, 2) through the drawing out of a minute 



Pele's Hair. (Krukenberg.) 



air-vesicle. They also show that the air- vesicles sometimes con- 

 tinued expanding as the glass was drawn out ; and that the 

 hair is often enlarged about enclosed crystals. The crystals are 

 rhombic, as in the figures. The facts make it evident that the 

 glass is far from being pure glass. 



3. THE EFFECTS OF THE EXPANSIVE FORCE OF VAPORS WITHIN THE LAVAS: 

 VESICULATION AND ITS MECHANICAL EFFECTS. 



a. Origin. — Yesiculation, the making of bubble-like cavities 

 in a melted rock, is a noiseless unseen effect of the vapors that 

 are rising and expanding within the lavas. The expansion 

 necessary to produce them is resisted by the cohesion in the 

 lava, and by the pressure. Consequently it is a very common 

 feature of the easily fusible volcanic rock, basalt, but not of 

 trachyte or rlryolyte, except in pumice, the glassy scoria of 

 these rocks ; and even this glass (obsidian) commonly holds to 

 its moisture, if it contains any, without vesiculating. 



Owing to superincumbent pressure, the maximum depth of 

 vesicles is small, as has long been recognized ; but how small 

 in basalt, or any other rock, has not been ascertained by ex- 

 periment. It probably does not occur in the Hawaiian Islands 

 below a depth of 200 feet. Above the lower limit, vesicles 

 may increase in number and size toward the surface, and be 

 largest in the scum or crust, as within Kilauea ; but this varia- 

 tion upward is not always a fact. 



o. Kinds. — Five styles of vesiculation may be 'distinguished 

 in the Kilauea ejections, two of which characterize stony 

 lavas, and three scorias. 



(1) That of the ordinary lava-stream of the floor of the pit. 

 The vesicles are oblong and of irregular shape, and constitute 

 from less than 1 to 50 or 60 per cent of the mass of the rock. 

 The form is spherical when the vesicles are very few and small. 



(2) That of the common stony spherically-vesiculated lava. 

 The vesicles make 30 to 60 per cent of the mass, and are too 



