J. D. Dana — History of the Changes in Kilauea. 225 



The vesicles of the finer kind are mostly g'^th to T \th of an 

 inch in diameter, like those of the 1880-'81 Mt. Loa flow; but 

 their walls are reduced to threads corresponding to the edges 

 of polygonal vesicles. Figure 1 shows the general appearance 

 of the Surface in a magnified view. The forms of the skele- 

 ton polygonal cells are, for the most part, either 12-sided or 

 14-sided figures, having a perimeter of ten or twelve pentag- 

 onal faces in two alternating rows, and bases of five or six sides. 

 The 12-sided "cells are bounded by the edges of pentagonal 

 dodecahedrons such as come from' the mutual pressure of 

 spheres, except that they are distorted usually by compression, 

 and by elongation or abbreviation. The 14-sided, which are 

 much the most common, are similar to the 12-sided in 

 general form, but have hexagonal bases. Fig. 2 is a side view 

 and fig. 3 an end view of one of the latter kind, and fig. 4 

 shows a group of such cells, as seen over the surface of the 

 scoria (a cut or broken surface, for it is impossible to handle a 

 piece of the scoria without breaking off bits of the brittle 

 threads). Fig. 6 is another of the 14 sided kind of less sym- 

 metrical form, as is common. One of the pentagonal dodeca- 

 hedrons is shown in fig. 7, and another in fig. 8. 



There is often a more complex system of network through 

 other crossing contour-threads, but the simpler forms are 

 referable to those represented. The inside of the base of one 

 of the large and therefore less regular forms is shown in fig. 5, 

 the diameter was about /o-th of an inch. In the largest vesi- 

 cles the walls are openly reticulated. 



The threads of this thread-lace scoria are not rounded, but 

 9. parts of the contours of the three 



elliptical cells that were there in 

 contact; and fig. 9 shows a por- 

 tion of one. Having this form, 

 'the glassy material of the threads 

 is thickest, and therefore of dark- 

 est color, at the center ; and they 

 are still thicker and darker at 

 the angles or junctions of three 

 threads. This glassy scoria calls to mind the vesiculation of an 

 obsidian by a high heat, converting it into pumice or scoria 

 because of its occluded water, as illustrated by Professor Judd, 

 and also by Mr. Iddings in experiments with the obsidian of 

 the Yellowstone Park. The Kilauea glass must have been 

 penetrated molecularly with water to have produced such a 

 result. Its ejection took place after the violent projection of 

 great stones ; and apparently not long after, as it overlies directly 

 the layer of stones. The conditions of origin in the cases 

 about the summit of Mt. Loa I cannot give. But the descrip- 



