﻿part 2] 



PICRITE-TESCHENITE SILL OF LTJGAK. 



87 



by a series of six west-north-west and east-south-east faults (see 

 fig. 1, p. 86).. At the north-eastern extremity of the outcrop the sill 

 is cut by a west-south-west and east-north-east fault which severs 

 it from another mass of igneous rock, extending half a mile back 

 towards the west. This mass, however, is on a higher horizon, 

 and has been mapped as separating the ' Millstone Grit ' from the 

 overlying Coal Measures. 1 If it be taken as part of the Lugar sill, 

 the total length of outcrop becomes 3| miles. 



The finest sections are found just above the confluence of the 

 Bellow and Grlenmuir Waters to form the Lugar Water. Both 

 streams have eroded deep rocky gorges through the sill, the one in 

 a north-easterly, the other in a south-easterly direction. When the 

 water is low, practically every foot of the thickness can be examined 

 either in cliff-section, or in horizontal water-polished areas of rock. 

 In these circumstances the study of the sill can be conducted 

 with facilities unattainable in any of the other occurrences ; and 

 the conclusions as to the origin of the different facies arrived at in 

 this case may be considered sufficiently well founded to apply to 

 the other occurrences, in which, although the exposures are not 

 so good, practically the same structure and disposition can be 

 made out. 



(1) The Glenmuir Section. 



The Grlenmuir Water, cutting through the sill in a general 

 north-westerly and south-easterly direction at right angles to the 

 strike, gives the most complete and typical section. The upper 

 contact is seen at the weir, just at the confluence with the 

 Bellow Water. Hardened whitish sandstone occurs overlying a 

 dense basaltic facies, in a steep rocky bank on the south side of the 

 river. The chilling influence of the contact, as shown by fineness 

 of grain, extends down about 12 feet, and has doubtless been 

 strengthened by the inclusion in the sill at this depth of a thin 

 band of sandstone now metamorphosed to a hard white quartzite. 

 The contact-rock is distinctly banded in layers, often confused, 

 wavy, and bifurcating, which differ slightly in colour and texture. 

 The thickness of the bands varies from several inches to very 

 fine linear streaks but faintly indicated by a slight difference of 

 colour. Besides the normal, greyish-black, aphanitic contact-rock, 

 the chief varieties included in the banded material consist of fine- 

 grained, pinkish, and greenish teschenitic rocks, and a very dense, 

 dead-black, glossy, basaltic material, although extremely slight 

 differences of colour and texture serve to bring out the banded 

 structure. These varieties show no sharp contacts with each other, 

 the transition from one to the other taking place quickly but 

 quite gradually. These bands seem to be true schlieren, clue to the 

 flow of a slightly heterogeneous magma. In general the streaks 

 are drawn out in bands parallel to the upper margin of the sill. 

 In addition to the banding, the contact-facies is traversed by 



1 See the Sheets of the 1-inch Map of the Geological Survey of Scotland, 

 Nos. 14 (1868) & 15 (1870). 



