Lawson.] 



Malignite. 



355 



the orthoclase, and there are occasional quite separate grains of the 

 albite in the slides, which are doubtless genetically connected with 

 that of the microperthite. These are the only plagioclase feldspars 

 in the slides. 



The dark green matrix in which these large idiornorphic 

 orthoclases (microperthite) are imbedded is a hypidiomorphic gran- 

 ular aggregate of aegerine-augite, melanite, biotite, titanite, and 

 apatite, stated in the order of their abundance, together with the 

 subordinate proportion of microperthite and albite above alluded 

 to. As accessor)' minerals occur magnetite and hematite in exceed- 

 ingly sparing quantities in a few slides. Of these minerals the 

 titanite and apatite are commonly idiornorphic, although occasionally 

 they are quite irregular in outline and are allotriomorphicly inter- 

 locked with the other minerals. The three principal constituents, 

 segerine-augite, melanite, and biotite, seem to have crystallized 

 simultaneously and are entirely allotriomorphic, although the 

 pyroxene shows the usual tendency to elongation parallel to c. 



The negerine-augite has the same optical characters as those 

 already described for its occurrence in the nepheline-pyroxene- 

 malignite. In some few cases, however, it is of a less deep green 

 color and may indeed be nearly colorless. These are rather excep- 

 tional occurrences, and are doubtless due to a local poverty of the 

 ajCTerine molecule. Twinning on x P m is common. The melanite 

 is in the form of reddish brown to yellowish grains, which are 

 strictly isotropic. They are quite fresh and contain no inclusions 

 as a rule. They have the high refractive index of the garnets and 

 their characteristic irregular cracks. A coarse powder of the rock 

 on being treated with hydrofluoric acid for 24 hours dissolved all 

 the other minerals and left the melanite intact. 



The biotite is a brown variety, shading into green locally, and 

 calls for no especial comment. 



The titanite appears in thin section in the form of brownish 

 yellow crystals, which commonly exhibit the characteristic rhombic 

 sections of that mineral. These crystals are usually prominent 

 features of the slides and attain a size of 3 mm., though commonly 

 they are about 1 mm. in length. They are more abundant than the 

 proportion of titanic acid found in the analysis would lead one to 



