70 G. W. Tyrrell — Alkaline Igneous Rocks, West Scotland. 



affinities of the intrusion of Necropolis Hill, Glasgow.' Many details 

 of geological occurrence and petrography are to be found in Sir A. 

 Geikie's Ancient Volcanoes of Great Britain."^ Numerous notes have 

 been contributed by tlie Geological Survey during recent years. 

 Analyses of the Barshaw theralite and the Lennoxtown essexite 

 have been made ; ^ whilst the occurrence of nephelitie in some of these 

 rocks was recorded by Bailey in 1909.* The teschenites of the 

 vicinity of Glasgow have been described by Macnair,* and the 

 teschenite-picrite sill of Lugar by Boyle. ^ A classification of 

 the Post-Carboniferous igneous intrusive rocks of the West of 

 Scotland was given by the writer in 1909, but that classification 

 must now be amended and supplemented in many particulars.' 



Peteography. 



A brief systematic description of the main types of intrusive rocks 

 and of the Mauchline lavas is here given. Much raineralogical detail 

 of great interest and importance and also the detail of field geology is 

 necessarily omitted or greatly curtailed, but will be given at length 

 in the detailed papers which it is hoped will soon be published. 



The intrusive rocks may be classified broadly into three groups — 



A. Rocks with conspicuous analcite. 



B. Bocks with conspicuous nepheline. 



C. Bocks without conspicuous analcite or nepheline, but which 



may contain either or both as accessory constituents. 



A. Rocks with conspicuous Analcite. 

 These may be subdivided into five groups as follows : — 



1. Analcite-syenite, in which an alkali-felspar is predominant. 



2. Teschenite, in which a lime-soda felspar is predominant. 



3. Picrite-teschenite, an ultra-basic difi'erentiation facies of 



teschenite. 



4. Lugarite, a leucocratic rock with affinities to ijolite. 



5. Monchiquite, a lamprophyric sub-group. 



Of these the teschenites are by far the most abundant. 



1. Analcite-si/enite.^ — This exceedingly rare rock-type is found in 

 a remarkably fresh and perfect occurrence at Howford Bridge, near 

 Mauchline, Ayrshire. It is exposed in the cliffs of tlie Biver Ayr, 

 and forms a lenticular intrusion into the lavas and tuffs which 

 underlie the ' Permian ' red sandstones of the district. In hand- 

 specimens the rock is medium to fine grained and whitish in colour. 

 It consists mainly of well-shaped felspars with a general parallel 

 arrangement giving a trachytoid aspect to the rock. The long 

 acute -angled interspaces are filled with greenish analcite and 

 dark ferromagnesian minerals. The most prominent constituents, 



' British Petrography, 1888, p. 194. 



- Vol. ii, pp. 58-67, 1897. 



■^ Suniviari/ of Progress of Geological Survey for 1907, 1908, p. 55. 



■» Ibid, for 1908, 1909, p. 44. 



^ Trans. Glasgow Geol. See, vol. xiii, pt. i, pp. 56-86, 1907. 



" Ibid., vol. xiii, pt. ii, pp. 202-23, 1908. 



' Ibid., vol. xiii, pt. iii, pp. 298-317, 1909. 



^ Nature, vol. Ixxxii, p. 188, 1909. 



