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ME. Ot. W. TTE.EE LL ON THE 



[vol. Ixxii, 



teschenite which pierce the contact-rocks, presumably after the 

 solidification of the latter ; and by the corrosion and replacement 

 suffered by the earlier minerals. The origin of the veins, irregular 

 dykes, and sheet of lugarite, is also probably to be referred to this 

 late stage in the history of the sill. This rock, in all probability, 

 represents the extreme analcitic term of differentiation effected in 

 the magma-chamber, injected later along the same channel as 

 the teschenite and picrite. 



(11) Comparison of the Lugar Sill with the other 

 Picrite-Teschenite Sills of Scotland. 



Including the Lugar sill six picrite-teschenite sills have been 

 described from the Midland Yalley of Scotland. The other locali- 

 ties are Ardrossan and Saltcoats in the west ; and Blackburn, 

 Barnton, and Inchcolm in the east. 1 Those that have been 

 described in any detail show marked correspondences with the 

 Lugar sill. There is always a central ultrabasic stratum, flanked 

 towards both contacts by teschenite, or by dolerite of teschenitic 

 affinities (save in the Blackburn sill where the base is not seen). 

 The upper band of teschenite is usually the thicker (Lugar, 

 Ardrossan, Barnton). In only two cases, Lugar and Blackburn, 

 is there a sharp contact between the upper teschenite and the ultra- 

 basic stratum ; and for these sills some degree of liquation has 

 been postulated. In the others the two types of rocks, where the 

 relation is observable, are said to pass gradually one into the other. 

 The differentiation in this case may be ascribed simply to the 

 gravitational settling of olivine. In the Blackburn, Barnton, and 

 Inchcolm sills, the ultrabasic rock is a picrite in the original sense 

 of Tschermak, an olivinic differentiate from teschenite. and still 

 contains a little felspar and analcite. At Lugar and Ardrossan 

 the gravitational action must have been effective for a longer 

 period, as a felspar-free hornblende-peridotite has been formed, 

 very rich in olivine ; and at Lugar, increasing richness in olivine 

 from the upper to the lower part of the stratum has been observed. 

 Flow-banding at both contacts has been observed at Lugar, but at 

 Ardrossan and Inchcolm it ajDparently occurs only near the upper 

 contact. At Barnton the central picrite shows a rude banding 

 parallel to the contacts. 



In two cases, Ardrossan and Inchcolm, the respective observers 

 have postulated heterogeneity in the magma prior to intrusion. 

 At Lugar also, both teschenite and picrite were slightly hetero- 

 geneoiis before intrusion, as shown by schlieren differing in mineral 

 composition or texture. The process of liquation has been invoked 

 to explain a sharp plane of separation between teschenite and an 

 underlying ultrabasic stratum in the Blackburn sill. 3 The influence 



1 For references, see pp. 84-85. 



2 ' The Geology of the Neighbourhood of Edinburgh ' Mem. Geol. Surv. 

 Scotland, 1910, p. 281. 



