122 G. W. Tijrrell — Alkaline Igneous Rock-s, We.si Scotland. 



The massive sill of Benbeoch, near Dalmellington, is taken as 

 the type - occurrence. Macroscopiccilly the rock is compact, fresh, 

 phanerocrystalline, and rather fine-grained. The dominant tint is 

 grey or greenish grey, the latter when olivine is very abundant. 

 In thin section the rock is composed of dominant olivine in fresh 

 rounded crystals, and titanaugite forming large anhedral to sub- 

 hedral plates and prisms much indented by the terminations of laths 

 of labradorite. ?Jany small rounded grains of olivine and some of 

 ilmenite are poikilitically enclosed in the augite. Labradorite and 

 subordinate nepheline form a kind of groundmass to the somewhat 

 larger-sized ferro-magnesian constituents, which tend to form poly- 

 somatic groups, and thus take on a pseudo-porphyritic aspect. Some 

 later felspars are zonal and range in composition from Ab^ Aug to 

 Abg Ang. Orthoclase may be detected on careful examination, 

 bordering the lime - soda felspar. The nepheline is somewhat 

 decomposed, but is recognized by the usual distinctive characters 

 (p. 79). A little clear analcite was the last constituent to crystallize. 

 Tlie only other minerals seen are skeletal ilmenite associated with 

 scraps of reddish biotite, and a little apatite. The general fabric is 

 evenly granular. The rock has been analysed by Dr. Dittrich, of 

 Heidelberg, with the result given in column I, Table IV below. 

 Four other analyses are tabulated for comparison. 



Table IV. 



Sp. gr. 



3-05 



3^065 



3^148 



III. 



IV. 



KyUte, Benbeoch, Dalmellington. 



Hornblende-picrite, Ty Croes, Anglesey. Bonney, Q.J.G.S., vol. xxxix, 



p. 256, 1883. 

 Ultra-femic olivine-basalt, lava-flow of 1852, Mauna Loa, Hawaii. Daly, 



Journ. Geol., vol. xix, p. 296, 1911. 

 Nepheline-basalt, Uvalde County, Texas. Cross, Bull. U.S. Geol. Surv., 



No. 168, 1900, p. 62. 

 Theralite, Flurhubl, Duppau, Bohemia. F. Bauer, T.M.P.M., vol. xxii, 



p. 281, 1903. 



