122 B. K. EMERSON — PLUMOSE DIABASE AND PALAGONITE 



anhydrous and like the black parts of the rock from Seljadalr, although 

 the latter was only brownish black. I was thus nonplussed when Mr 

 George Steiger, who had separated the pure material from my Holyoke 

 occurrence with the greatest skill and patience, announced that it con- 

 tained 17 per cent of water. 



I then had slices cut from speimens of the " palagonitfels " of Seljadalr 

 (see plate 30, figure 4), which came to the Amherst collection directly 

 from Professor Steenstrup, of Copenhagen, and they seemed to me to 

 be not clastic in any proper sense, but to be portions of a flow some- 

 what brecciated by steam explosion in place, and an examination of the 

 two good-sized hand specimens confirmed this opinion. Though quite 

 friable they seem wholly homogeneous, and swarms of small pores con- 

 tinue for long distances through the mass and fade away into the com- 

 pacter portions, while the dark color in the same way fades away into 

 the resin yellow parts. The whole seems to me not a tuff, but a much 

 cracked glass, containing a very few inclusions of a deep red glass and 

 of an indeterminate basalt, and very rarely a minute crystal of augite 

 or plagioclase. The original glass was a fawn color and is still regularly 

 scattered in remnants, often angular in the general mass, but more than 

 half the mass has suffered a slight alteration to a bright yellow glass, 

 sometimes slightly polarizing, and a similar layer is found around all 

 cavities. There is also a thin layer of a brightly polarizing fibrous devit- 

 rification substance lining each cavity inside the yellow layer. The 

 peculiar irregular structure of the yellow glass is due to its being made 

 up of collapsed steam holes. Threads of glass, the interior of these vari- 

 ously collapsed and distorted steam holes, the interior of the unaltered 

 steam holes, as well as the small solid spherulites, all show the same 

 brightly polarizing fibrous layer, often several times repeated, and of 

 closely the same thickness. This seems to me to be an original structure 

 in the sense in which the fibrous structure of spherulites is original, and 

 to have been produced by the influence of the H 2 present at the instant 

 of cooling. This water hydrated the glass later for a further distance out 

 from the cavities, changing the original fawn-colored glass to yellow. The 

 repeated expanding and collapsing of the steam has often minutely shat- 

 tered the glass, producing a micro-brecciated effect, but tracing from one 

 end of the slide to the other the glass is essentially continuous, and I can 

 detect no later microchemical cement, no true fragmental structure, nor 

 any dull pulverulent interstitial matter like what one finds in an ordi- 

 nary palagonite-bearing tuff, as, for instance, that from the Aschen 

 Kuppel near Giessen, from which I have slides. This shows, even with 

 the naked eye, its tuffaceous character, while the two fine large blocks 

 of the Seljadalr rock seem with a strong lens a cracked but homogeneous 



