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THORODDSEN 



streams. The tuffs have usually a brownish-yellow colour owing to 

 the intermixture of palagonite, a brown dully-lustrous alteration- 

 product of tachylyte or basalt-glass, which constitutes the greater 

 portion of the ground-mass of the rock ; therefore the whole forma- 

 tion is often called the palagonite formation. The tuffs consist of 

 lava-dust and lava-fragments with an abundance of glass-pieces (pala- 

 gonite, tachylyte), slag and bombs; loose anorthite-crystals are often 

 abundant. The breccias are distinguished from the tuffs bv beins 



U %s 



Fig. 5. Cliffs 011 KlificS (tuff and breccia). The Veslmannaeyjar. 



more coarse-grained and containing larger, angular lava-pieces: the 

 fragments consist of compact basalt, dolerile, pumice, slaggy lava 

 and volcanic bombs: the separate fragments are often covered with 

 a glassy crust. The palagonite formation is sometimes arranged in 

 layers; sometimes not a trace of regular arrangement can be dis- 

 cerned. The tuffs and breccias have been formed by volcanic erup- 

 tions which chiefly ejected ashes and lava-fragments, and produced 

 only a few lava-streams; the ruins of the numerous large volcanoes 

 from which these eruptions proceeded may still be demonstrated. 

 The palagonite formation is traversed by thousands of basalt dykes 

 which branch and send out apophyses and intrusive sheets; very 

 often the breccia and the tuff is filled with numerous hollow nodules 

 and balls of basalt with a radiallv-columnar structure inside and 



