64 BUNSEN ON THE FORMATION OF 



compositions of the latter are almost exactly one and the same, 

 the chemical relation of the zeolitic rocks to the form which 

 they originated, can no longer be ascertained by means of calcu- 

 lation. However, even a superficial observation of the geogno- 

 stic relations in which they occur, leaves'no doubt as to the nature 

 of their formation. Near Silfrastadir, as well as at innumerable 

 other places in Iceland, the zeolitic amygdaloids, having some- 

 what the character of conglomerates, are seen to pass through 

 gradual transitions on the one side into solid trap rock, and on 

 the other into palagonitic tuff, so intimately blended indeed that 

 the concretions and fissures may be traced from the trap through 

 the amygdaloid and into the tuff rocks. At Silfrastadir, where 

 the trap, rising in rocky precipices above the beds of tuff, admits 

 of a closer examination of these relations, the formation of zeo- 

 lites appears most perfectly developed at the contact of the two 

 kinds of rock characterized by this gradual transition, and de- 

 creases towards the compact rock in proportion as the visible 

 traces of a mutual action disappear more and more, so that finally 

 it is only in fissures and isolated cavities that the fine druses of 

 chabasite are found which are characteristic of the amygdaloid 

 formations* of that locality. This phaenomenon occurs every- 

 where in Iceland. It may be observed even in the most recently 

 erupted lavas. One of the most remarkable examples of this 

 kind occurs on the Krafla. The frequently more arenaceous than 

 tuffaceous beds of this volcano, if indeed a range of tuff moun- 

 tains penetrated by craters and lava and traversed by furma- 

 roles may be so termed, are intersected upon the north-western 

 declivity of the mountain by what appears to be a very recent 

 lava, which has issued not from the opening of a crater, but from 

 horizontal fissures in beds. The neighbouring palagonitic rock 

 has, from the point of contact with these beds of lava, suffered 

 a most remarkable metamorphism, which may be best observed 

 under a microscope of thirty or forty-fold magnifying power. 

 The substance of the anhydrous rock, without exactly having 

 been melted, is separated into two siliceous masses, of which one 

 is dark and ferruginous, and the other brilliantly white and 

 free from iron. The former appears as a homogeneous matrix in 

 which the latter is imbedded ; both are amorphous. Nearer 

 towards the lava, where the igneous action was more consider- 



