THE COMPOSITION OF KULAITE 6l$ 



true that many of these are markedly lower in Si0 3 , A1 3 3 and 

 Na 2 0, but at the same time analyses of these are to be found 

 which closely resemble mine of the Kula rocks, as those of the 

 nepheline-tephrite from the Falkenberg, near Tetschen, Bohemia, 

 described by Hibsch (VIII), and the leucite-nepheline-tephrite 

 of Niedermendig (IX). 



They also resemble closely the monchiquites, as shown by a 

 typical analysis of one of these (X), the main difference being 

 in the higher content of H 3 in the monchiquites, consequent 

 on their containing analcite in place of nepheline. 1 



Rocks which chemically closely resemble these are certain 

 " basalts " from Colorado, described by Cross, an analysis of one 

 of which is given in XI. These carry considerable orthoclase, 

 but no nepheline, and with biotite instead of hornblende. 



Turning to the mineralogical composition, it must be noted 

 that most of the Kula rocks present a peculiarity which adds 

 somewhat to the difficulty of classification. They have been 

 spoken of as containing hornblende as an essential constituent* 

 This is strictly true only of the more glassy varieties, in which 

 basaltic hornblende is fresh and unaltered. In the others- — 

 including' those analyzed- — this mineral has been partially or 

 entirely altered to hypersthene, diopside and magnetite, so that, 

 speaking with strict accuracy as to their present composition 

 the majority of them cannot be said to contain any hornblende 

 at all. 



This naturally raises the difficult question, whether the orig- 

 inal hornblende should be recognized as a component. Of its 

 initial presence, and the derivation of the hypersthenic and 

 diopside-magnetite "mixed crystals" or pseudomorphs from it 

 there can be absolutely no doubt, from the evidence of the 

 shape of the pseudomorphs, the transitions to unaltered horn- 

 blende, and other facts observed here and elsewhere. 



The case is strictly comparable with the occurrence of 

 so-called " pseudo-leucites," mixtures of orthoclase and nephe- 

 line or muscovite, pseudomorphous after original leucite. As 



'Cf. L. V. Pirsson : Jour. Geol., Vol. IV, p. 679, 1896. 



