F. D. Adams — New Nepheline Rock. 273 



the syenite holds scapolite. Taking the intrusion as a whole 

 it may be said to consist essentially of albite, nepheline and 

 hornblende, but as is so frequently the case in this region it 

 shows a wide variation in relative proportion of the constituents 

 in different places. In some places it is rich in feldspar, while 

 elsewhere the nepheline almost entirely replaces this mineral. 

 In the latter case the nepheline is usually associated with a 

 considerable amount of hornblende, in addition to which in 

 many cases a small amount of red garnet is present. 



The feldspathic and feldspar-free varieties ran in rudely 

 parallel bands or schlieren. These are often several feet in 

 width and may be traced for several hundred yards along the 

 strike of the banding. They represent distinct magmas result- 

 ing from extreme differentiation. The rock here described was 

 collected from one of the bands six feet in width and several 

 hundred yards long. 



The rock is coarse in grain and consists essentially of white 

 nepheline and black hornblende, the former preponderating 

 largely. It thus has a rather striking appearance on the fresh 

 fracture. On the weathered surface the contrast presented by 

 the two minerals is less striking, as the nepheline assumes a 

 pale gray color. Under the influence of the weather, the 

 nepheline, as is always the case with these rocks in this dis- 

 trict, presents the appearance of having been dissolved away, 

 the weathered surface being smooth and recessed, the horn- 

 blende and the accessory feldspar and cancrinite of the rock 

 standing out from the surface of the nepheline. 



Under the microscope the rock is seen to consist essentially 

 of nepheline and hornblende, with plagioclase, cancrinite, and 

 calcite as accessory constituents, as well as sodalite, apatite, 

 sphene, biotite, p} 7 rite and iron ores, these latter minerals being 

 present in extremely small amounts. 



The Nepheline occurs in large well-defined grains, present- 

 ing the usual characters displayed by the species. It is clear 

 and fresh. 



The Hornblende is green in color, the pleochroism and 

 absorption being as follows : a= pale greenish yellow, b and c= 

 very deep green. . The absorption is c=b>ct. The maximum 

 extinction observed in the sections of the rock was 19°. It is 

 an alkali hornblende, containing less iron than hastingsite, 

 but, like it, as shown by the calculation of the analysis of the 

 rock, belonging to the division of the Syntagmatites. 



The Plagioclase is present only in very small amount and is 

 in some cases untwinned, while in other cases it shows a faint, 

 polysynthetic twinning. In thin sections it bears a very close 

 resemblance to the nepheline, and when untwinned it is diffi- 

 cult in all cases to distinguish the two minerals. When a sec- 



