PALAGONITE OF SELJADALR 123 



resinous glass, which shows over considerable areas a finely porous 

 texture certainly original. A few cavities in the mass are filled with a 

 colorless mineral of very low refraction and cubical cleavage sometimes 

 slightly polarizing, which is doubtless analcite. 



The material resembles the Holyoke glass in the appearance of the 

 collapsed steam holes and the fibrous inner devitrification layer, but in 

 its hardness (4.5), color, and greater amount of lime it is more like the 

 basal beds described by me from Greenfield and Meriden, and it seems 

 probable that some part of the palagonite rock, including the specimens 

 I have studied, had the same origin, namely, they were formed as a sub- 

 marine flow over a bottom made up of volcanic ashes, and the water 

 sometimes explosively penetrating from below the thin crust which had 

 formed beneath the lava and rising up into and blending with the molten 

 mass, caused the sudden formation of glass full of collapsed and distorted 

 steam holes and carried up the foreign fragments of partly decomposed 

 basalt, sideromelan (tachylite), augite, and olivine (and perhaps, also, 

 the shells and infusoria sometimes found in the mass), into the interior, 

 as can be seen distinctly to have happened in the walls of the trap ridges 

 at the localities at Meriden and Greenfield described above. On study- 

 ing the matter further I came upon the following points confirming 

 this idea : 



Bunsen, who first mentioned the locality at Seljadalr, described it as 

 a flow of glass 80 or 100 feet thick, in contradistinction to all the other 

 localities of the palagonite in Iceland, which he calls tuff beds. Von 

 Waltershausen describes it as almost wholly pure palagonite, " with few 

 points of sideromelan," and calls it palagonitfels, though he does not 

 distinctly speak of it as a flow. This is in the same paper in which he 

 describes the sideromelan,* and he describes the latter as black, and 

 with a hardness of 6, and scarcely mentions it at Seljadalr; and as the 

 black parts of the latter rock have a hardness of 4 or 4.5, it is not prob- 

 able that the dark glass so abundant here (fawn-colored under the micro- 

 scope) is the same which he analyzed and found anhydrous, nor is it 

 possible that this dark glass is anhydrous. It is dark brown by reflected 

 light and olive green when seen with the lens in the thin-section with 

 transmitted light, and in this it agrees exactly with the " diabase pitch- 

 stone " from Meriden which I have described in the paper cited above 

 and which contained 4.72 per cent H ? 0. The darker and original por- 

 tions of the Icelandic glass may be not much more hydrous, while the 

 yellow parts may have become much more hydrous by absorbing the 

 water which frothed it up to produce the many collapsed steam holes. 



* Vulcanischer gesteine in Sicilien und Iceland, pp. 182, 202. 

 XVII-Buli.. Geol. Soc. Am., Vol. 1G, 1904 



