H. 8. Washington — New Leucite Rock. 37 



0-010 mm.) sub-spherical grains of a colorless, birefring- 

 ent substance, that is similar to that seen in the leucite. 

 These microlites are arranged in long lines, like strep- 

 tococci, some of the lines being straight and more or less 

 parallel, while others are curved. 



Mode. — As only one small, and probably not represen- 

 tative, thin section was available, no attempt was made 

 to determine the mode by Rosiwal's method. But from 

 consideration of the figures given by the norm and by 

 separations with heavy solution and the magnet, the 

 mode is estimated to be approximately as follows : 



Leucite 90 



Hauyne 2 



Melilite 2 



Pyroxene 4 



Melanite 1-5 



Biotite 0-2 



Magnetite 0-2 



Apatite 0-1 



1000 



It is to be supposed that the hauyne and melilite, 

 neither of which is visible in the section, form the glass, 

 the amount of which is estimated at about 4 per cent. 

 The presence of hauyne, melilite, and melanite is quite 

 in accord with the mineral composition of other highly 

 salic and potassic Italian lavas, which frequently carry 

 small amounts of these minerals. 



Chemical composition. — For the chemical analysis, 18 

 grams (about one-fourth) of the specimen was crumbled 

 to fragments over a large sheet of paper, and the whole 

 was reduced to powder and thoroughly mixed. It is 

 thought that it represents fairly well the composition of 

 the specimen, though, of course, a larger sample from 

 much more material would be preferable for such a 

 coarse-grained rock. The analysis was made according 

 to the usual methods, the alkalies and sulphur trioxide 

 being determined in duplicate. The results are shown 

 in Column 1 of the Table, there being also given some 

 other analyses of highly leucitic rocks. 



