﻿part 2] PICKITE-TESCHENITE SILL OE LUGAE. 85 



The famous picrite of Inchcolrn, in the Firth of Forth, is well 

 known from the descriptions of several observers. Dr. R. Campbell 

 and Mr A. Stenhouse, however, in a recent detailed investigation 

 of the island, have shown that at both the upper and lower 

 contacts the picrite passes into teschenite. 1 On the west coast, 

 Dr. J. D. Falconer has described a picrite-teschenite sill at Ard- 

 rossan, intrusive into the Carboniferous Limestone Series : here 

 again the ultrabasic rock occupies the central part of the mass. 3 

 Recently the late R. Boyle drew attention to still another occur- 

 rence at Lugar, near Old Cumnock (Ayrshire). He described the 

 passage of dolerite and basalt [teschenite] through doleritic picrite 

 [theralite] to ' segregated ' masses of peridotite or picrite. The 

 ultrabasic rock occurs in the central parts of the mass, and passes 

 gradually to less basic varieties towards both upper and lower 

 contacts. 3 A picrite associated with teschenite has been dis- 

 covered by Mr. E. M. Anderson at the Inner Nebbock, Saltcoats 

 (Ayrshire) . 4 



In the course of an investigation of the Permo- Carboniferous 

 alkalic rocks of the West of Scotland 5 I made a detailed exami- 

 nation of the Lugar sill, and found in it an extraordinary complex 

 of various rocks belonging to the analcite series. This included 

 normal and melanocratic teschenites ; a facies with abundant 

 nepheline and ferromagnesian minerals — essentially a melanocratic 

 theralite ; and a curious rock composed mainly of analcite and 

 nepheline, with subordinate plagioclase, titanaugite, and barkevikite 

 in very perfect crystals. This unique rock, which it is proposed to 

 call lugarite, has now been found in several localities in the West 

 of Scotland. Extremely fresh hornblende-picrites and peridotites, 

 however, form the major part of the intrusion. 



The present paper embodies a complete description of this sill, 

 and attempts an explanation of the processes whereby the different 

 facies have been developed. A comparison with the other occur- 

 rences is also instituted. Five chemical analyses have been made 

 in the course of the investigation, and in connexion with these and 

 for the work in general I have to acknowledge the aid of a grant 

 from the Government Grant Committee of the Royal Society. For 

 the analyses I am indebted to the skill of Dr. Alexander Scott, of 

 Glasgow University. 



II. Field Relations. 



The intrusion which is the subject of this paper occurs near the 

 village of Lugar in Central Ayrshire, near the eastern border of the 

 county. It is intruded as a sill into a crumbling white and yellow 



1 ' The Geology of Inchcolrn ' Trans. Geol. Soc. Edin. vol. ix, pt. 2 (1908) 

 pp. 121-34. 



2 ' The Geology of Ardrossan ' Trans. Roy. Soc. Edin. vol. xlv, pt. 1 

 (1907) pp. 601-10. 



3 Trans. Geol. Soc. Glasgow, vol. xiii (1908) pp. 202-23. 



4 ' Summary of Progress for 1911 ' Mem. Geol. Surv. 1912, p. 52. 



5 Geol. Mag. dec. 5, vol. ix (1912) pp. 69-80, 120-31. 



