G. W. Tyrrell — Alkaline Igneoua Rocks, West Scotkoid. -125 



The alkali-dolerites and essexite-dolerites are abundantly developed 

 in tlie Dalraellington district. The chief localities are Muck Burn, 

 Dalnean Hill, Dalcainnie Hill, Grimmet Glen, Whitehill (Patna), 

 Polnessan Burn, Kankinston, Dunaskin Glen, and Ashbeugh Glen. 

 In the district of which Tarbolton is the centre, the chief localities 

 are Auchincruive, Helenton, Howford Bridge, Dippol Burn, Trabhoch 

 Burn, Stair, Failford, Auchinweet, and Barskimming. 



The an alcite- syenite differentiate is only found in the Mauchline- 

 Tarbolton district ; it lias not been recognized in the Dalmellington 

 area. In the latter, however, the alkali-dolerites sometimes contain 

 pink acid veins, notably at Muck Burn, Dalmellington, and the Coyle 

 Water, Rankinston. At the latter locality the veins consist of 

 a medium-grained plexus of zonal plagioclase margined by orthoclase, 

 associated with an approximately equal quantity of moire and 

 perthitic orthoclase in broad anhedral plates. A little granular 

 pyroxene and skeletal ilmenite is enclosed in the felspars, also some 

 biotite and chlorite ; but the proportion of femic minerals is very 

 small. This rock has affinities with the monzonites. 



The Muck Burn variety is much finer in grain, and attains a vein- 

 width of 5 inches. It consists of a fine-grained aggregate of well- 

 shaped laths of orthoclase and acid oligoclase, the latter slightly 

 predominating. The interstices are filled with quartz and crypto- 

 crystalline matter. The only other constituent is a little leucoxene. 

 The structure is typically orthophyric, and the rock might be termed 

 a monzonitic orthophyre. If the orthoclase is soda-bearing, as is 

 probably the case, the rock becomes a keratophyre. 



D. The Maucliline Lavas. 



These rocks occur in a narrow ring-shaped outcrop surrounding the 

 'Permian' red sandstones of the Mauchline-Tarbolton area. Their 

 geological features have been described by Sir A. Geikie.^ Their 

 petrographical character is the subject of a brief note \)j Dr. Hatch in 

 Sir A. Geikie's Ancient Volcanoes.- The picrite there described is 

 probably one of the monchiquite or limburgite lavas. Mr. David 

 Perguson, who has devoted much attention to this district, estimates 

 the thickness of the lavas at 280 feet. This thickness is made up by 

 a number of thin flows intercalated with tuff and red sandstone. 

 These intercalations increase in number and thickness towards the 

 top of the lavas until they merge into the thick overlying sandstones. 

 The lavas have been poured out from a series of small ' greenhill ' 

 vents situated mostly round the northern edge of the outcrop.^ 

 Numerous vents also occur round the margin of the high Carboni- 

 ferous plateau to the north of Dalmellington, which was doubtless 

 once covered by a cap of lavas. The vents contain lavaform material 

 in blocks and apparently intrusive masses, which extend our knowledge 

 of the petrography of this late series of Palaeozoic lavas. 



^ Geol. Mag., Vol. Ill, p. 243, 1866. 

 ^ Ancient Volcanoes of Great Britain, vol. ii, p. 57, 1897. 

 ^ Tyrrell, " The Geology of Mauchhne," in The Land of Burns, by J. T. 

 Gibb, of Mauchline, 1911. 



