G. W. Tyrrell — Alhaline Igneous Rocks, West Scotland. 73 



The analysis shows that this analcite- syenite belongs to the group 

 of the alkali-syenites, and not to the felspathoidal group. The 

 Madagascan ditroite quoted does not contain nearly so much alkalies 

 as the usual run of nepheline-syenites, and seems rather to belong to 

 tVie alkali-syenites. The comparatively high lime content in the 

 Howford Bridge rock, due to the abundance of pyroxenes, is to be 

 noted, as it is a point of difference with the majority of the alkali- 

 syenites. This feature links the rock to the umptekites, but no very 

 concordant > analysis of the latter could be discovered. Amongst 

 previously described Scottish Carboniferous rocks, the phonolitic 

 trachytes of East Lothian, notably that of Berwick Law, show 

 decided chemical affinities with the Howford Bridge rock, as may be 

 seen by comparing Analysis IV with I. These rocks are said to 

 contain analcite and probably pseudoraorphs after nepheline.' 



Towards the base of the Howford Bridge sill the proportions of 

 titanaugite, iron-ore, and labradorite increase rapidly, and the rock 

 passes into an essexite-doleiite identical with those described later 

 (in Part II). The contact facies is a dense splintery basalt of similar 

 composition. A thin layer of essexite-dolerite also forms the upper 

 contact of the sill, but it passes rapidly downwards into the normal 

 analcite-syenite. The sheath of essexite-dolerite surrounding the 

 analcite-syenite is penetrated by numerous ii-regular anastomosing 

 veins of the latter rock. It is probable that in this sill we have 

 a case of gravity differentiation similar to that of the Lugar (pp. 75, 76) 

 and Benbeoch {infra) masses. 



Another occurrence of analcite-syenite is known from a sill 

 intruding the ' Permian' lavas in the Dippol Burn, IJ- miles north, of 

 Ochiltree. Here, however, the analcite-syenite occurs in thin con- 

 temporaneous stratiform bands in an essexite-dolerite which forms the 

 major part of the sill. The rock is generally similar to that described 

 above, but the rhomboid form of the felspars and the trachytoid 

 structure is accentuated. Veins of analcite-syenite occur also in the 

 essexite-dolerite of the Trabbocb Burn, near Stair (see Part II of paper). 



Analcite-syenite of a different type is to be found in the numerous 

 aplitic veins which penetrate many of the teschenites. These are 

 fine-grained light-coloured rocks forming veins up to 2 inches in 

 width. In the Necropolis (Glasgow) teschenite the veins are 

 composed of a plexus of plates and idiomorphic prisms of orthoclase, 

 sometimes moii'e and perthitic, embedded in an abundant groundmass 

 of dusty analcite. A little analcitized labradorite may also be present. 

 Some flakes of a bleached biotite and a few skeletal crystals of 

 ilmenite are the only ferromagnesian minerals to be found. In one 

 slide, however, there are a few needles of segirine, and some sections, 

 the shape and mode of decomposition of which suggest the presence 

 of nepheline. The texture is fine and even-grained, and the rock 

 might be termed analcite-syenite-aplite. Veins occur in many other 

 teschenites, notably the Galston type, in which they are quite coarse- 

 grained hypidiomorphic syenites. 



2. TcKchenite. — Rocks belonging to this group are very abundant, 

 and are fairly uniform in character. The sill of the Necropolis Hill, 

 ^ Mem. Geol. Surv., Geology of East Lothian, 1910, p. 130. 



