G. W- Tyrrell — Alkaline Igneous Rocks, West Scotland. 121 



and resembles neither the essexites of Lochend and Craigleith in the 

 Lothians, nor the Crawfordjohn type described below. It is poor in 

 alkali-felspars and felspathoids, is devoid of purple augite, and has 

 a well-developed trachytoid fabric. 



(b) Crawfordjohn type. — This type, which was first described by 

 Teall, is named after the famous locality for curling stones in South 

 Lanarkshire. The type-rock apparently comes from one of the long 

 north-west-south-east dykes which are believed to be of Tertiary age. 

 It is, however, so exactly identical with rocks from Lennoxtown and 

 Patna, and so incongruous with the Tertiary suite that there seems 

 no alternative than to regard the rock as belonging to the Late 

 Palaeozoic alkaline types. The Lennoxtown occurrence forms a 

 massive irregular dyke intrusive in Calciferous Sandstone lavas on 

 the southern slopes of the Campsie Hills ; the Patna occurrence is 

 a small boss intruding the agglomerate of the Late Palaeozoic vent of 

 Carclout Hill, and therefore establishes the age of this type. The 

 latter occurrence is congruous with the presence of alkaline intrusions 

 in other vents of the same district, and the Late Palaeozoic age of 

 the entire suite seems to be well established. 



As the Crawfordjohn and Lennoxtown rocks have been previously 

 described by Allport, Teall, and Bailey, only the new occurrence at 

 Patna falls to be described here. In hand-specimens it is greyish, 

 compact, fine-grained, carrying numerous lustrous black phenocrysts 

 of augite. Microscopically it is composed of a plexus of broad laths 

 of plagioclase (Abj^ -^"i)) which are idiomorphic to abundant and 

 often beautifully fresh nepheline. Limpid analcite occupies some of 

 the interspaces and encloses the euhedral terminations of felspar laths. 

 Titano-magnetite is aggregated into groups of euhedral crystals 

 associated with flakes of biotite. In the above, as groundmass, 

 numerous large phenocrysts of deep purple titanaugite and fresh 

 olivine are set. The pyroxene moulds and encloses olivine, and 

 occasionally also the terminations of the felspars. In size it ranges 

 up to half an inch in diameter. The olivine rules much smaller, and 

 should perhaps be associated descriptively with the groundmass. 

 Apatite occurs in great abundance enclosed in nepheline and analcite. 

 Nepheline occurs in the Lennoxtown and Crawfordjohn rocks, as 

 described by Lacroix,^ and Bailey.* It is sometimes, as in the Patna 

 rock, so abundant as to make it a question whether the rock should 

 not be described as a theralite. Mr. Bailey, however, has placed the 

 Lennoxtown occurrence in the essexite family on account of its 

 very close resemblance - to Brogger's essexite from Brandberget, 

 Christiania district.* 



3. Kylite. — This well-individualized type occurs as a homogeneous 

 and abundant set of sills and bosses in the Kyle district of Ayrshire, 

 from whence it derives its name. It is an olivine-rich ultra-femic 

 theralite or essexite. The chemical and mineralogical composition is 

 quite distinctive ; and as the rock is not a mere diiferentiation-facies 

 of theralite or essexite, but forms numerous independent sills, it 

 has been thought advisable to mark its individuality by giving it 

 a new name. 



^ Compte Rendu, cxxx, 1900, p. 1273. ' Bailey, loc. eit. 



