386 J. J. H. Teall—Nepheline- Syenite in N.W. Scotland. 



district, appear to be of sufiScient interest to justify a brief preliminary 

 notice, which it is the purpose of this communication to supply. 



The work of previous observers on the rocks in question is 

 described in the paper on borolanite above referred to, and need not 

 therefore be repeated in detail ; but there is one name I desire 

 especially to mention in the present connection, namely, that of 

 Dr. Heddle. In one of his papers on the geognosy and mineralogy 

 of Scotland ^ he gives chemical analyses of the porphyritio felspars 

 and of the groundmass of a " hornstone porphyry " from the south- 

 east spur of Ben Brachaid. Both agree very closely with albite in 

 composition. Thus the credit of establishing the existence of rocks 

 formed by the consolidation of an alkaline magma exceptionally rich 

 in soda, in the Assynt district, unquestionably belongs to Dr. Heddle. 



The post-Cambrian igneous rocks of the north-west of Scotland 

 occur in two forms — as plutonic masses and as sills and dykes ; 

 or, in other words, as abyssal and hypabyssal rocks. No lavas 

 or effusive rocks of any kind are known. The only important 

 plutonic mass is the one stretching from Ledbeg for about jfive miles 

 in a south-easterly direction, with an average width of about one 

 mile. Cnoc na Sroine (1,306 feet) forms the culminating point. The 

 dominant rock of this mass is a red granite or syenite, remarkably 

 free from ferro-magnesian or any other dark-coloured constituents. 

 It was described by Nicol as a red felspar porphyry, and by 

 Murchison as a syenite. Dr. Heddle pointed out that true 

 porphyritic structure is absent,^ and that the rock is composed 

 mainly of red felspar, often much stained and decomposed ; quartz 

 being always subordinate to the felspar and sometimes absent 

 altogether. 



The specimens in the Survey collection agree with Dr. Heddle's 

 descriptions. Those in which quartz is present have the aspect 

 of somewhat coarse-grained binary granites, while those in which 

 it is absent resemble syenites. Ferro-magnesian constituents are 

 extremely rare in the main mass of Cnoc na Sroine, but they occur 

 abundantly in some of the peculiar rocks found on the margins. 

 A specimen from the bum behind the inn at Aultnacallagach 

 (3,082) ^ is a coarse-grained red granite or quartz-syenite, composed 

 mainly of felspar, but containing also a few blebs of quartz and 

 some insignificant dark specks representing a ferro - magnesian 

 mineral. Under the microscope two felspars are recognizable. The 

 plagioclase occurs in more or less idiomorphio crystals, which are 

 often zoned and always twinned on the albite plan. The twin 

 lamellse are numerous and very narrow. The second felspar occurs 

 in large, irregular plates, and shows moire-structure under crossed 

 nicols. It is often twinned on the Carlsbad plan, and frequently 

 contains inclusions of idiomorphio plagioclase. Quartz occurs in 



* Mineralogical Magazine, vol. v (1884), p. 141. 



''A specimen in the Survey collection (3,081) from the summit of Cnoc na 

 Sroine shows a half-porphyritic structure such as is occasionally found on the 

 margin of Brogger's nordmarkite. 



^ The numbers in parentheses refer to the registered specimens in the collection of 

 the Geological Survey. 



