Alnoitic Rocks at Isle Gadieux, Que. 5 



positive; in somewhat greater amount it is about iso- 

 tropic, and where present in considerable amount it is 

 zoned, with the inner zones nearly isotropic and the outer 

 zones distinctly negative. 



The perovskite is quite abundant for a constituent that 

 usually occurs in such minor amounts. It is in sharp 

 octahedra of a wine yellow color in thin section and is 

 apparently isotropic, the twinning phenomena not being 

 visible in these grains. 



The opaque ore mineral has a bluish reflection and is 

 presumably titanif erous magnetite ; at any rate it is 

 strongly magnetic. It occurs rather abundantly as dis- 

 seminated grains and is particularly strongly developed 

 as a rim about corroded chrysolite grains where they 

 abut against biotite, monticellite or melilite. 



The apatite is present in unusually large amount and 

 in relatively large grains. It is distinguished from meli- 

 lite by its higher birefringence and by the hexagonal 

 shape of its basal sections. 



The marialite occurs as a few small grains that were 

 detected in only some of the slides where they were 

 closely associated with monticellite. Its refringence is 

 comparable with that of quartz, that is, only slightly 

 higher than that of balsam, and in this particular it is in 

 marked contrast with all the other constituents of the 

 rock which are uniformly minerals of high relief. The 

 birefringence is likewise comparable with that of quartz, 

 but is appreciably higher. It is uniaxial and negative. 

 These properties agree with those of a scapolite close to 

 the sodic end of the series, that is, marialite. This 

 mineral has been observed in trachytes of the Phlegrean 

 Fields, near N-aples. 



The alteration products were not given very careful 

 study. They are mainly carbonates formed by the 

 alteration of monticellite and melilite. The carbonate 

 patches are frequently crowded with minute needles of 

 apatite which suggest that the alteration of the minerals 

 is not to be assigned entirely to weathering but is in part 

 a post-magmatic phenomenon intimately connected with 

 the consolidation of the rock. It is noteworthy that 

 neither olivine shows the serpentine type of alteration. 



"While the rock as a whole is considerably altered, the 

 alteration is unevenly distributed, and in many parts of 



