IIG C. H. GORDON — SYENITE-GNEISS (lEOPARD ROCk) FROM CANADA. 



The size of the large microcline grains is, in general, proportional to that 

 of the feldspathiclump. The titanite occurs chiefly in association with 

 the au<>'ite. It appears also in the feldspathic areas, but less frequently. 



In thin section the microcline grains of the feldspathic lumps are found 

 to measure from I2 millimeters to 2 millimeters in diameter, while the 

 granular matrix in which they lie is composed of grains of microcline and 

 plagioclase varying in size from .3 of a millimeter]to .6 of a millimeter. 



The small grains of microcline in the granular areas often ap])ear 

 cloudy at the center, while the outer portion is clear and fresh. The 

 same appearance is also observable in some of the plagioclase grains. 

 In these and in many other sections the granular feldspars are often seen 

 to meet each other in strai<^ht, sharply defined boundaries, suggesting 

 an approach to crystallo^raphic outlines. This tendency is especially 

 pronounced in grains in which the outer zone is fresher tlian the inte- 

 rior. The larger microcline grains are filled with particles of epidote, 

 plates of biotite, fluid inclusions, and occasionally microscopic grains of 

 tourmaline. They are generally much fractured and in some cases 

 show cracks filled with calcite. Quartz, calcite and plagioclase feldspar 

 occur in the augite, often, but not al\va3^s, in connection with fractures. 

 Small aggregates of hornl)lende needles, accompanied by "an opaque iron 

 ore, appear occasionally as alteration products of the augite. The latter 

 often shows distinct pleochroism: 



a=greenish yellow. 



b= green. 



r=green or slightly yellowish green. 



Quartz is not ])lentiful. 



In nuin1)er 143 (150, 151, 152, 153) the ellipsoidal masses are larger, but 

 are more or less flattened and contain a considerable amount of quartz. 



Under the microscope the interstitial areas show a pronounced devel- 

 opment of the granular or " mortel structure," in which in some places 

 there ap[)ears a preponderance of plagioclase beset with an abundance 

 of nodular quartz, while in others granular microcline predominates. 



The constituents have the relations characteristic of the micropoikilitic 

 structure. The larger grains of microcline sometimes show the eff'ects of 

 dynamic agencies in the bending of the lamelUe and undulatory ex- 

 tinction. The augite occurs in irregular grains and aggregates distrib- 

 uted along the middle of the granular band, as usual. 



Quartz is quite abundant in angular, often elongated grains. They 

 sometimes inclose small, rounded grains of pyroxene, which in one case 

 appeared to stream out from a large augite grain adjoining. 



The apatite occurs, as usual, in rounded and elongated grains, some- 

 times inclosino- rounded grains of augite. 



