44 H. S. Washington — New Leucite Rock. 



regular through mutual interference during growth. It 

 is interesting to note that the leucite forms a granular 

 aggregate, quite analogous to its occurrence in the intru- 

 sive missourite, discovered by the late Prof. Pirsson. 14 



The leucite shows the twinned structure well, but not 

 as remarkably as does the italite of Rocca Moniina. 

 There is no regular arrangement of inclusions. The 

 leucite is perfectly clear and unaltered. 



Scattered through the rock, and especially included in 

 the leucite, are many rather small crystals of augite. 

 These are mostly euhedral, showing prism and pyramid 

 planes. They are of an olive-yellow color, not pleo- 

 chroic, with some tendency to zonal structure, the ex- 

 tinction angle about 45°, and in general quite like the 

 typical augites of most of the Italian leucite rocks. 

 They carry no inclusions, and the augite is, with the mag- 

 netite, evidently the first mineral to crystallize. 



Interstitial between the leucite grains, and evidently 

 the last mineral to crystallize, is considerable melilite, 

 which forms thick, tabular subhedra, with well-marked 

 basal cleavage, and showing the characteristic blue-gray 

 between crossed nicols. This mineral will be described 

 in detail later by Dr. Buddington. With this fresh meli- 

 lite, and apparently an alteration product, are similar 

 forms of what looks like melilite, but which does not give 

 the blue-gray color, though it has also extinction parallel 

 to the base. This is usually fringed on the edges with 

 narrow strips of a finely granular decomposition prod- 

 uct, of high birefringence, that resembles hydronephelite 

 or ranite. 



There are very few small magnetite grains, an apatite 

 prism here and there, but no biotite, olivine, melanite, 

 nor mineral of the hauyne group was seen. There is no 

 glass, and evidently the melilite takes its place, though 

 not so markedly interstitial as is the glass in the italite 

 proper. 



From examination of the norm and the thin sections 

 the mode is estimated to be approximately as follows : 



Leucite 60 (60) 



Melilite 23 (18) 



Pyroxene 16 (20) 



Magnetite 1 (2) 



100 



14 This Journal, (4), 2, 315, 1896; U. S. Geol. Survey, Bull. 237, 115, 

 1905. 



