226 J. D. Dana — History of 'the Changes in Kilauea. 



tions seem to imply that the spongy scoria there is one of the 

 results of high jettings or fountain-like throws of the lava dur- 

 ing an eruption ; it may be a light form of ordinary scoria. 



The minute delicacy and brittleness of the threads in this 

 scoria suggests a way of making fine dust by volcanic action, 

 which is much more reasonable than that of mutual friction of 

 projected fragments of scoria of the ordinary kind ; it thus 

 helps in the understanding of the lofty dust clouds of Krakatau 

 and Tarawera. 



c. Amount of moisture required for vesicidatio?i, its distri- 

 bution, and its origin. — The facts derived from the crowded 

 vesiculated lava of 1880-1881, reaching from its source down to 

 Hilo, over 30 miles, and throughout the whole range remarka- 

 ble for uniformity and for depth in the stream, besides giving 

 an opportunity to study the origin of the vesiculation and the 

 amount of moisture it requires, presents also evidence as to the 

 origin of the moisture in the conduit and its condition. 



(1) As I learn from Rev. E. P. Baker, the vesicles change 

 little toward the summit except in becoming coarser, with 

 thinner walls, at the source. From the mean size, Jj inch in 

 diameter, we obtain for the size of the particle of moisture 

 required at the ordinary pressure to fill one of the vesicles, 

 •000,000,007 of a cubic inch. What the size actually was, 

 under the pressure and the temperature that existed at the 

 time of vesiculation, cannot be determined. But this much we 

 learn, that the moisture was distributed throughout the lava in 

 a state of extreme division, actually or essentially that of 

 molecular diffusion. 



(2) The space in the vesicles is 40 per cent of the mass, 

 as determined from the specific gravity of the rock-material, 

 2 '98, and that of the mass with the surface varnished to exclude 

 the water, 1*88. The required water is hence '0003 per cent 

 of the mass ; or by weight '0001 per cent ; showing that, the 

 amount of water reqxiired for the vesiculation is exceedingly 

 small. 



From the thread-lace scoria we find, since only 1*7 per cent 

 of the mass is solid glass, that the amount of moisture required 

 to produce the vesiculation, at the ordinary pressure, would 

 be 3 - 125 per cent of bulk, and l'l per cent by weight The 

 amount of moisture was hence not unusual for a rock, although 

 the vesicles occupied 98*3 per cent of the mass. 



(3) The source of the flow of 1880, 1881, according to Mr. 

 Baker, was about 11,100 feet above the sea-level. This is 

 2575 feet below the summit of Mt. Loa, or about 1600 feet 

 below the bottom of the summit crater. Before the outbreak, 

 the liquid lavas were active within the crater; that is, the 

 length of the conduit above the place of outbreak was then 

 about 1800 feet. On account of the pressure of 1800 feet 



