326 



the fact that quartz possesses slightly higher double-refractive 

 power is also useful. 



The dark mica is more abundant in certain wavy planes which 

 sweep round the tumid lenticles above referred to. A pale-coloured 

 mica occurring in good-sized crystalline plates and containing zircon 

 inclusions, surrounded by pleochroic borders, also occurs. Ill-defined 

 crystals and granular aggregates of garnet are not uncommon in the 

 gneissose rocks. 



The gneissose rocks differ from the massive rocks so far as they 

 have been examined (1) in the presence of a larger amount of 

 granulitic material, (2) in the presence of a larger amount of light- 

 coloured mica, and (3) in the presence of garnet. The large grains 

 of felspar in the gneissose rocks almost always show microcline 

 structure, whereas those of the massive rocks are often unstriated or 

 only striated after the fashion of ordinary plagioclase. In some of 

 the finer-grained rocks the whole of the quartzo-felspathic material is 

 present in the granulitic condition. Flakes of dark mica, or of dark 

 and white mica, are then seen to lie in a cranulitic asfOTea-ate of 



O OO O 



quartz and felspar. The mica-flakes are as a rule much broader than 

 the individual grains of quartz and felspar. Such rocks might be 

 conveniently termed mica-granulites. An interesting question arises as 

 to the origin of the gneissose structures. That they are due to the 

 plastic deformation of a mass of granitic material seems perfectly clear. 

 The only point open to discussion is whether the deformation was 

 produced during or subsequent to consolidation. The author inclines 

 decidedly towards the latter view. 



In the Islands oft' the west coast of Scotland we have, as is well 

 known, granites and felsites of Tertiary age. (1) Professor ZIRKEL < 2 > has 

 given us some details as to the petrographical characters of these 

 rocks, and Professor JUDD has described their geological relations. The 

 red rocks of Skye, termed syenite by the older authors, were named 

 quartz-syenite and hornblende-bearing or mica-bearing folsite-porphyry 

 by Professor ZIRKEL. They form an extensive tract lying between 

 Sligachan and Broadford. Dykes of basic trap abound in the district ; 

 some of these are cut off by the granitic and felsitic rocks, whereas 

 others penetrate these rocks. 



The granitic and felsitic rocks are composed, according to ZIRKEL, 

 of orthoclase, plagioclase, quartz, hornblende (here and there some mica), 

 magnetite, apatite and felsite. By felsite he means a substance which 

 cannot be resolved by macroscopic examination. When the felsite is 

 absent we have a fine-grained holo-crystalline rock composed of 

 the above-mentioned minerals. This rock ZIRKEL terms quartz-syenite. 

 The ferro-magnesian minerals do not enter largely into its composition 

 and quartz is generally abundant. The rock is of acid composition. 

 It is not a sub-acid rock as the name syenite appears to imply. 



(1) JUDD. Q.J.G.S., Vol. XXX. (1874), p. 220. 



(2) Z.D.G.G., 1871, p. 1. 



