HEAT. 



281 



few vesicles to the bulk of the rock, and the scoriaceous varieties contain 

 many, so many as to make the rock light. Pressure of much overlying lava 

 prevents vesiculation, and this takes place, therefore, near 

 the surface ; but it is not ascertained what amount of pres- 

 sure so limits it. 



241. 



If 



f 



Ordinary vesicles are usually oblong, rather than spherical, unless 

 the size is quite small ; no distinction between those made in volca- 

 noes by different kinds of vapors has been observed. But in some 

 streams of igneous origin (as in the trap of the Connecticut Valley) 

 they sometimes have the form of slender cylinders, 2 or 3 inches long ; 

 and such elongated forms imply great expansive action at the moment 

 of vaporization, and therefore point to the vaporization of liquid car- 

 bonic acid as the cause. 



Oblong vesicles sometimes are pointed at one end, and thus show 

 the direction of movement — that of the blunt end ; an example from Kiama, New South 

 Wales, is here represented. 



Vesicles in basalt 

 Kiama. D. '49. 



at 



The lightest of all kinds of scoria, called "thread-lace scoria," has the 

 thin walls reduced to mere threads, as in the annexed figures of a specimen 

 obtained at Kilauea. Fig. 242 represents a portion of the scoria, magnified 

 30 times. Figs. 243 and 244 show two forms presented by the more regular 

 of the cells. The form of Fig. 243, which has 12 sides besides the two bases, 



is the most common. 



The natural size of the cells is -^ to ^ inch, though 



242, 



244, 



Thread-lace scoria, from Kilauea. x30. D. 



some are much larger. This scoria contains only 1-7 per cent of its bulk 

 in rock-material, and hence a layer of glass one inch thick would make a 

 layer 60 inches thick of the scoria ; and 1*2 per cent of moisture in the 

 glass by weight would suifice to produce it. 



The light glassy scum of the lava of the Kilauea lava-lakes (like the scum on ferment- 

 ing molasses) flows off as the top of each outflowing stream, and cools as a separable 



