RESUME 125 



3. The cavities may be isolated or so abundant as to froth consider- 

 able areas into a porous mass. 



4. The expansion of the gas may be explosive and the collapse imme- 

 diate, and the microscopic crystalline pellicle may be brittle, and its parts 

 crack and slip on each other (plate 29, figures 1 and 2), or plastic and 

 wrinkled into the shrunken cavity like a wet cloth (plate 28, figure 2), 

 still very clearly showing its identity and differentness from the inclos- 

 ing glass by the fibrous and brightly polarizing character. 



5. The H 2 is then slowly absorbed into the glass, and so far as it 

 goes the pale brown glass is changed into a bright yellow slightly 

 polarizing glass. 



6. This change, extending outward from several adjacent centers, 

 leaves angular, often concave-sided remnants of the brown glass which 

 exactly simulate lapilli. It is also possible that the repeated small ex- 

 plosions might occur when part of the glass had already solidified and 

 it be shattered and the fragments moved and enveloped in the still 

 liquid glass. 



7. The H 2 has expanded in a swarm of pores, which are elongated 

 by flow and then lined by a perfectly even layer of the fibrous crystal- 

 line material. This crystallization is thus plainly subsequent to the 

 appearance of the H 2 as a liquid and can not be called on to explain 

 the isolation of the H 2 by the formation of an anhydrous silicate in the 

 hydrated glass after the manner of explanation of the lithophysse pro- 

 posed by Professor Iddings. Indeed, when I examined the lithophysse 

 in the Yellowstone their size and abundance seemed to me to demand 

 some more abundant source than the glass itself for a part at least of the 

 H 2 0. Here I should derive at least the larger part of the H 2 from the 

 sea bottom and think of it as introduced with the foreign tuff fragments 

 found in the mass either simply by a picking up of the wet material as 

 the lava flowed over it or by more local and explosive penetration of the 

 thin solid bottom layer which would naturally form and generally pro- 

 tect the liquid mass from the wet bottom. 



Discussion by Alfred R. Lane 



When Professor Emerson was kind enough to show to the Society at 

 Washington his specimens of " plumose diabase," with the large arbo- 

 rescent growths of augite, I was very much interested and examined 

 them rather particularly, the more so because in one point they led my 

 mind to an inference different from that which Professor Emerson 



