J. J. H. Teall — Nepheline- Syenite in N. W. Scotland. 391 



represented either in the main mass or in its immediate neighbour- 

 hood. But the resemblance extends still further to some of the 

 hypabyssal rocks — the dykes and sills. A glance at the maps 

 (Sheets 101 and 107) will show that the Cambrian area to the north 

 of the plutonio mass is traversed by numerous sills and dykes. 

 These belong to two strongly marked types — dark - coloured 

 hornblendic rocks (camptonites or vogesites) and light - coloured 

 felspathic rocks (felsites). A description of the hornbleudio rocks 

 has already appeared in the pages of this Magazine/ and they need 

 not further be referred to at present. A full description of the 

 felsites must be postponed until analyses are available, but one or 

 two varieties may be briefly described, as they are clearly allied to 

 certain peculiar rocks occurring in the Christiania district. 



The first of these is a dyke from Poll an Droighinn, near 

 Inchnadampf (2,324). The rock is a pale-green felsite, with 

 somewhat obscure indications of porphyritic felspars and decided 

 traces of parallel structure. Under the microscope it is seen to 

 be composed of polysynthetic aggregates, i-epresenting original 

 porphyritic alkali - felspars, streaks of microcrystalline quartz 

 (scarce), and a crypto- or microcrystalline felspathic matrix 

 croioded with acicular microlites of CEgirine. Similar microlites occur 

 in the polysynthetic aggregates, as they do in the phenocrysts in 

 Professor Brogger's grorudite, but they are far less numerous than 

 in the matrix, where they are often so thickly crowded together 

 as to form a felt-like mass. The larger microlites are green, but the 

 smaller ones are colourless ; both show the characteristic optic 

 characters of asgirine when isolated from the felspathic material. 

 This rook is a variety of grorudite. 



Another very interesting type of felsite, found by Mr. Peach, is 

 intrusive in the Lewisi9,n gneiss in a burn one-quarter of a mile 

 north of the top of Sgonnan More (8,370). This consists of 

 numerous phenocrysts of pink felspar, usually giving rectangular 

 sections and measuring about a quarter of an inch across, embedded 

 in a compact light-grey felsitic matrix. Under the microscope the 

 phenocrysts are seen to consist of intergrowths of albite and 

 orthoclase, similar to those observed in the plutonic mass of 

 Cnoc na Sroine. The groundmass is a micro- or cryptocrystalline 

 aggregate of alkali-felspar, containing a few minute, ragged prisms 

 of aegirine. A little free quartz is probably present, but cannot 

 be identified with certainty under the microscope. This rock 

 contains much less ^girine than the typical grorudites, and in this 

 respect more closely resembles the lindoites of Professor Brogger. 



Analyses of these rocks will be made, and until this has been 

 done a further discussion of their affinities seems scarcely desirable. 

 This much is certain. They are composed mainly of alkali-felspar 

 with some free quartz and segirine, and they are closely allied to 

 grorudites, from which they difi'er chiefly in containing rather less 

 segirine. 



' " Notes on eome Hornblende-bearing Rocks from Inchnadampf," by J. J. H. 

 Teall: Geol. Mag., 1886, pp. 346-353. 



