H. S. Washington — New Leucite Rock. 43 



It will be noted that the garnet of Beaver Creek, and 

 Magnet Cove, which contain (the former variably) 

 considerable titanium, show indexes much like that of the 

 Rocca Monfina mineral; while the ivaarite of Finland, 

 which is almost black and opaqne, and has the largest 

 amount of Ti0 2 , shows the highest index. The melanite 

 of East Rock, in which Dana conld detect no titaninm, 

 is distinctly birefringent, and therefore the low tempera- 

 ture form of garnet, 13 and shows the lowest refractive 

 index. As will be recalled by all students of mineralogy 

 at Yale, this melanite occurs in crevices in the "trap," 

 not as a constituent of the rock itself, so that the fact 

 that it is the low temperature form, and consequently 

 birefringent, is not surprising. 



Vesbite (melilite italite). 



While this paper was being written, Dr. A. F. Budding- 

 ton, of the Geophysical Laboratory, called my attention 

 to a specimen of melilite that he had obtained from 

 Ward's Natural History Establishment. The melilite 

 cystals are in crevices and small cavities in an "ejected 

 block" from Monte Somma. While it is, in general, not 

 very satisfactory to describe a rock type from such a 

 specimen, yet, in this case, the rock is so very peculiar 

 and so strikingly like my specimen of italite, that it seems 

 to merit description. 



The rock is light gray, noncrystalline, and very clearly 

 an effusive igneous rock, and not a metamorphosed block. 

 It is rather coarsely granular, made up for the most part 

 of grains of very light gray leucite, of irregular but gen- 

 erally equant form and closely packed together, these 

 grains measuring from 3 to 5 mm. in diameter. With 

 them is a white mineral in smaller amount, the melilite 

 revealed by the thin section. Very small and fairly 

 abundant black prismoids of pyroxene are scattered 

 through the leucite, and give the grayish color to the 

 specimen. The rock is somewhat vesicular, and tabular, 

 white melilite crystals occur in the vesicles, most of the 

 cavities showing signs of some weathering of their con- 

 tents. 



Thin sections show that the rock is made up of nearly 

 75 per cent of clear, fresh leucite, in closely packed 

 anhedra, some of them spheroidal and others quite ir- 



13 Cf . H. E. Merwin in C. W. Wright, U. S. Geol. Survey, Prof. Paper No. 

 87, p. 107, 1915. 



