G. W. Tyrrell^ Alkaline Igneous Rocks, West Scotland. 71 



however, are large black, lustrous crystals, the lustre of which is 

 interrupted by metallic patches which themselves become lustrous 

 when the rock is held in a different position to the light. These 

 lustre-mottled aggregates are composed of titanaugite^ and ilmenite, 

 and occur in increasing abundance towards the base of the sill. In 

 an acid variety of the rock which occurs in contemporaneous 

 stratiform bands and veins ranging up to 2 feet in thickness these 

 augite-ilmenite aggregates are absent. 



In thin section the rock is found to be composed principally of 

 alkaline felspars (anorthoclase, orthoclase, albite) and very sub- 

 ordinate labradorite, with abundant interstitial analcite of perfect 

 freshness and limpidity. The felspars and analcite make up the 

 great mass of the rock. The ferro-magnesian constituents include 

 titanaugite, segirine, brown and blue soda-amphiboles, ilmenite, and 

 occasionally a very little olivine. The texture is medium-grained, 

 and the fabric equigranular to trachytoid. 



In the analcite-syenite proper anorthoclase is the dominant felspar. 

 It occurs in well-shaped square or rhomboid sections frequently 

 showing the minute microcline twinning characteristic of this mineral. 

 It is usually bordered by a broad zone of orthoclase. Albite occurs 

 as occasional perthitic intergrowths, as independent idiomorphic 

 crystals, and as a product of the ' albitization ' of the labradorite. 

 It is nevertheless not abundant. In the more basic types labradorite 

 forms broad zonally - built crystals ranging in composition from 

 Abj Adj to Abg Anj. 



These felspars are loosely aggregated together and appear as if 

 floating in the abundant analcite which fills all the interspaces. 

 The latter is fresh and limpid, and shows beautiful cross-hatching 

 similar to that of leucite. Numerous idiomorphic crystals of segirine 

 and of brown and blue soda-amphibole (barkevicite and arfvedsonite) 

 are enclosed in the analcite. These are frequently found in confused, 

 tangled crystal aggregates at one side, or in one of the angular 

 bays, of an analcite area. There can be but little doubt that these 

 crystals gravitated into the lower portion of the analcite area as the 

 latter was situated during the cooling of the magma. This fact, 

 together with the perfect idiomorphism of most of the constituents of 

 the rock, affords impressive testimony, not only of the long duration 

 and high degree of liquidity of the residual magma, but also of the 

 primary nature of the analcite. 



In the more basic varieties of the rock titanaugite is a prominent 

 constituent. It is of a variable purplish-brown tint, becoming green 

 on the margins where it adjoins an alkali felspar or an area of 

 analcite. It is frequently shot through with large skeletal crystals of 

 ilmenite in blebs, patches, and strings extending right through and 

 beyond the boundaries of the crystals. These form the large lustre- 

 mottled aggregates prominent in some hand-specimens. Apatite 

 occurs abundantly in small bifid crystals usually enclosed in the 

 analcite, 



^ This term is used throughout for augite with the purple-madder tint 

 supposed to indicate a high titanium and alkali content. 



