336 L. V. Pirsson — Geology of Bermuda Island; 



fragments in the powders and in a number of cases embedded 

 in the sections of groundmass in the form of grains or 

 aggregates of them. It is an alteration product of perovskite 

 and augite, and the more altered the rock the more abundant 

 it becomes. It is readily determined by its high index of 

 refraction, strong double refraction, small optic angle, and 

 marked dispersion of the optic axes, together with its color. 



Perovskite, in unaltered, identifiable form, was not found in 

 any section of the melilite-basalt, but the writer agrees with 

 Dr. Thomas in considering yellowish masses as indicative of its 

 former presence. 



Melilite. — Strong suspicion was aroused concerning the 

 possible presence of this mineral, both from the nature of the 

 rock and from the chemical analysis. The difficulty in detect- 

 ing it lay in the prevalent alteration which the groundmass 

 has undergone. For the most part the colorless cementing 

 substance consists of anal cite or indeterminable whitish 

 material thoroughly clouded with extremely minute specks and 

 granules of calcite, such areas giving an aggregate high polar- 

 ization effect. It is evident that the mineral substance has 

 suffered a process of calcitization. Dr. Thomas was more 

 fortunate in finding some yet unchanged areas in which it 

 could be recognized, and encouraged by this, new sections were 

 prepared and finally in one of them, from the sample mentioned 

 by him, 1045-1075 feet, melilite was found with the characters 

 he mentions. Between crossed nicols it has not been observed 

 to have any abnormal interference color, only a gray, like 

 apatite or nephelite. It is negative in these sections. The 

 alteration of melilite into carbonates, chiefly calcite, is not 

 mentioned by Rosenbusch, but has been recently described and 

 discussed by Soellner in his paper on bergalite, the remarkable 

 melilite rock from the Kaiserstuhl.* This would be a natural 

 form of alteration in a rock submerged in sea-water containing 

 C0 2 . The material is not suited for observing the alteration 

 of melilite into deeckeite described by Soellner. After the 

 characteristic melilite with its peg structure had been seen and 

 studied, it could be traced in many of the calcitized areas by 

 minute remnants of the peg structure still remaining. 



Nephelite, sodalite, Jiauynite. — Nephelite has been found in 

 several cases in isolated fragments in the slides made from the 

 rock powders, as recognized by its colorless character, negative 

 uniaxial interference figure, refractive index close to balsam, 

 and low interference color. Whether these fragments come 

 from the melilite rock or from the monchiquite-like type, since 

 the samples are mixtures, is uncertain, though it is thought to be 

 probably present in both. Sodalite and hauynite were tested for 

 * Mitt. d. Grossh. Bad. Landesanst, vii, pp. 438, 452, 1913. 



