124 G. W. Tiirr ell— Alkaline Igneom Eock.% West Scotland. 



This coiiche of ultra-basic rock in the centre of tlie Benbeoch sill 

 appears to be susceptible of precisely the same explanation as that 

 adopted for the similar phenomena of the Lnp;ar complex (p. 76) ; 

 and also for an olivinic ledge in the Palisade (juai'tz-diabase sill of 

 New Jersey.^ 



C. Hocks without conspicuous Analcite or Nepheline. 



Associated with the more alkaline types described above are a great 

 number of less alkaline rocks, which, however, betray their affinities 

 by the abundance of purple augite and the occasional presence of an 

 interstitial wedge of clear analcite. Some of them accompany the 

 analcite-syenites apparently as differentiation-facies. Others form 

 small masses intrusive in the highest horizons in the West of 

 Scotland — the ' Permian ' red sandstones and lavas of the Mauchline 

 district. While rather variable in their characters it does not seem 

 possible to divide them into well-characterized groups. The following 

 description applies particularly to the sill in the Trabboch Burn, near 

 Stair, Ayrshire, a fresh and typical example of these rocks which may 

 serve as a type of the whole suite. It contains a few veins and 

 irregular patches of analcite-syenite. 



Megascopically the Trabboch Burn rock is greyish in colour, medium 

 and even-grained, consisting essentially of grey felspar and black 

 pyroxene in approximately equal proportions. In thin section the 

 rock consists of a coarse-grained aggregate of broad laths of zonal 

 plagioclase in subophitic relations with large subhedral to anhedral 

 plates of purple titanaugite. Fresh olivine occurs in some quantity, 

 and also ilmenite, the large skeletal aggregates of which are inter- 

 grown poikilitically with the pyroxene. It is associated, as usual in 

 these rocks, with a few scraps of red biotite. A soda-bearing 

 orthoclase may occur in some quantity, generally with analcite. 

 The latter fills up interspaces and encloses the idiomorphic terminations 

 of felspars, grains of augite, and needles of apatite. The proportions 

 of orthoclase and analcite are very variable, and in some sections they 

 are absent. The plagioclase is highly zonal, ranging in composition 

 from Abj^ An^ to Abj Auo, and has often suffered partial analcitization. 



All the other rocks falling under this group have the same general 

 aspect as the above. There is, however, considerable variation in 

 the proportions of certain constituents. Analcite and orthoclase 

 frequently fail, olivine often becomes an abundant constituent, whilst 

 many of the rocks show a perfect ophitic structure. 



These rocks are ophitic dolerites, which, whilst not conspicuously 

 alkaline, show considerable affinity with the alkaline series. This 

 may be appropriately recognized \)\ terming them aJkali-dolerites. 

 Those varieties which contain analcite and soda-orthoclase belong to 

 the essexite family, and their doleritic aspect maj- be recognized 

 by the designation essexite-dolcrite. They closely resemble the 

 ' crinanites ' of Argvllshire. (Mem. Geol. Surv., Geologt/ of Colonsaij, 

 1911.) 



^ J. V. Lewis, Annual Report of State Geologist for 1907 : Geol. Surv. of 

 New Jersey, 1908, p. 129. 



