G. W. Tyrrell — Alkaline Igneous Rocks, West Scotland. 75 



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the contact facies of the Krdrossan picrite-teschenite sill. Mr. Bailey- 

 has described the rock as nepheline-teschenite, and regards it as allied 

 to the porphyritic essexite of Lennoxtown.' Some of the teschenitea 

 show leucocratic and melanocratic modifications. This is well seen in 

 the extraordinary complex of the Lugar teschenite-picrite (p. 77). 

 The leucocratic variety is a coarse-grained pinkish 

 rock rich in analcite and orthoclase, whereas in the 

 melanocratic varietj^ the felspar and analcite total 

 only about 20 per cent of the mass. In texture this 

 rock resembles the Cathcart rather than the Glasgow 

 type. 



3. Piente-teschenite. — In two or three cases the 

 differentiation of a basic teschenite has given rise to 

 a considerable body of ultra-basic rock. The most 

 remarkable instance of this occurs at Lugar in 

 Central Ayrshire.^ This intrusion, which is magni- 

 ficently dissected by the Bellow and Glenmuir 

 Waters, contains an extraordinary assemblage of 

 analcite and nepheline-bearing rocks. It is a sill 

 about 140 feet in thickness, and intrudes a white 

 sandstone belonging to the Millstone Grit. At both 

 upper and lower contacts is a black basaltic rock 

 grading insensibly, through fine-grained varieties, 

 into a typical coarse teschenite of the Glasgow type. 

 Below the upper teschenite comes about 10 feet of 

 beautifully fresh theralite with abundant nepheline. 

 This rock, of which there are two varieties, differs 

 from the Barshaw rock (p. 80) in several particulars, 

 and will be described under the head of theralite. 

 It rests upon a great thickness of picrite or peri- 

 dotite, which, in its turn, rests upon the lower 

 teschenite, but without the intervention of a theralite 

 zone. The ultra-basic rock occupies about five- 

 eighths of the total thickness of the sill (see Figure). 



In hand-specimens the picrite is medium- to 

 coarse-grained, blackish-green in colour, and fre- 

 quently exhibits flashing, lustre-mottled plates of 

 hornblende. Microscopically it is composed of 

 olivine in more or iess rounded grains, ranging up 

 to half an inch in diameter, and in all stages of 

 alteration to blue, green, yellow, and colourless 

 serpentine. It is often quite fresh and forms about 

 65 per cent of the rock. The next most abundant 

 constituent is titanaugite, which totals about 21 per 

 cent. It occurs as numerous, minute, euhedral 

 grains in little clusters or polysomatic groups wedged in between the 

 olivines, and where the latter is altered, enveloped in serpentine. Horn- 

 blende occurs to the extent of 10 per cent in large irregular plates 



^ Summary of Progress of Geological Survey for 1908, 1909, p. 45 ; also Mem. 

 Geol. Surv., Geology of the Glasgoio District, 1911, p. 131. 



2 Boyle, Trans. Glasgow Geol. Soc, vol. xiii, pt. ii, pp. 202-23, 1903. 



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