J. J. H. Teall — Nephellne- Syenite in N.W. Scotland. 387 



irregular grains. The ferro-magnesian mineral is represented by 

 minute scales of a bright green chlorite. 



When the powder of the rock is placed in a Sollas' diffusion 

 column three well-marked bands are formed : one corresponding 

 to quartz, which is present only in small quantity (2 -65) ; another 

 corresponding to albite (2'62) ; and a third at a level just below the 

 point at which orthoclase floats. Although the specific gravity of 

 the plagioclase corresponds very closely with that of albite, the 

 extinctions on M-flakes are not quite equal to albite — 15° to 17° 

 as against 19°. The difference is so slight that in the descriptions 

 which follow the term albite^ will be used. The second felspar, 

 being slightly denser than orthoclase, is probably rich in soda, and 

 may be cryptoperthite or anorthoclase, but the optical characters, 

 so far as I have been able to determine them, agree with orthoclase, 

 and in the absence of analyses I prefer to use that term. Of the two 

 felspars albite is the more abundant. 



Another specimen from Cnoc na Sroine (3,090) is very similar 

 in general appearance to the one just described, but contains less 

 quartz. Two felspars are recognizable under the microscope, but 

 they are more intimately intergrown, and often assume the character 

 of microperthite. Both are deeply stained with ferric oxide, and 

 opaque iron-ore occurs as an accessory constituent. 



Other red rocks from the same mass are true syenites without 

 quartz, and these sometimes contain pseudomorphs after nepheline 

 with accessory melanite (3,083). The felspars in these cases are 

 either orthoclase or microperthite. 



The more acid varieties of the red rocks have affinities with the 

 nordmarkites of Professor Brogger. They resemble them in the 

 abundance of alkali-felspars and in the paucity of both quartz and 

 ferro-magnesian minerals. When the two felspars are intergrown 

 microperthitically the resemblance is very close, but when the albite 

 is independently developed as idiomorphic crystals it is not so close. 

 Taking the mass as a whole, the alkali-granites or quartz-syenites 

 appear to shade into quartzless syenites, and these again into 

 nepheline-syenites. 



In the red rocks which form the main mass of Cnoc na Sroine no 

 fresh nepheline has been detected ; but a coarse-grained dark-green 

 rock (3,095) from the foot of the north slope of the mountain 

 contains this mineral in abundance, and is, in fact, a true uepheline- 

 eyenite. The weathered surface is rough owing to the more rapid 

 weathering of the nepheline, which has a dull green, waxy 

 appearance. The alkali-felspar is developed in flat tables with 

 conspicuous development of the clinopinacoid, and the crystals 



1 It may be of interest to mention that in the metamorphic rocks (albite -schists 

 and gneisses) I have always found the albite to give the theoretical extinction of 

 19° on M-flakes. The mineral appears to be perfectly homogeneous and ideally pure, 

 except as regards inclusions. But I have never observed the theoretical extinction 

 of 19° in a plagioclase occurring as a normal constituent of an igneous rock. In the 

 case above described the breadth of the band in the diffusion column, the zonal 

 Btructure seen in the sections, and the slightly varying extinctions on the M-flakes, 

 all indicate that the mineral is not perfectly homogeneous. 



