124 B. K. EMERSON — PLUMOSE DIABASE AND PALAGONITE 



Indeed, the single deep red fragment of glass found in the slide, as men- 

 tioned above, is more probably like the original sideromelan. It is 

 plainly foreign and like the tachylite of Sasebuhl or Sordavala. Pro- 

 fessor von Waltershausen figures and describes the palagonite as a great 

 sheet overlying a soft and fresh tuff,* and finds all the contents of this 

 tuff bed as scattered inclusions in the palagonitfels above. This is ex- 

 actly as in the Meriden and Greenfield examples, except here, as the 

 flow was over a rusty sand bed, the inclosures have a more foreign and 

 peculiar aspect. Indeed, in an early paper upon the palagonites Profes- 

 sor Rosenbusch admits the possibility that the palagonitfels of Seljadalr 

 may have been a lava now,f and says that the characteristics of the 

 glass and its alteration can be equally well understood on either sup- 

 position. 



This palagonite has been the subject of most peculiar theories which 

 one may recall briefly. Bunsen thought it due to the smelting of the 

 normal basalt with 13 parts of CaO or K 2 0, and imitated it by this pro- 

 cess. Von Waltershausen developed an elaborate theory of its forma- 

 tion from a submarine tuff bed, where largely by the action of the sea 

 water the adhydrous glass sideromelan was changed into a hydrated 

 porodine cement which united the whole in a solid mass. This cement 

 formed a series of hydrated feldspars. Professor Rosenbusch gave the 

 correct explanation of the origin and alternation of the palagonite and 

 its agglomeration in tuff beds, but seems to have chosen as a type of his 

 •'Aschentuff " one of the few true and rare flows of the material. 



. Resume 



We may distinguish the following steps in the consolidation of the 

 palagonite : 



1. At the beginning of the process a minute globule of superheated 

 H 2 caused the formation around itself of a radiate-fibrous spherulite, 

 brightly polarizing with black cross and positive sign, which may be 

 also concentric structured by slight oscillations in its rapid growth. J 



2. The same H^O may be able to expand, and then the radiate-fibrous 

 layer is produced by the same cause around the inside of the cavity. 

 The cavities may be spherical or drawn out by flow. The solid spheru- 

 lites may be deformed or mutually impress each other or several be in- 

 cluded in the continued growth, showing that they were formed in the 

 still plastic magma. 



*Loc cit., pp. 481,483. 



t Neues Jahr. f. Min., 1872, p. 165. 



\ It is necessary to free glass from gas bubbles to prevent devitrification. 



