﻿part 2] 



PICRITE-TESCHEJflTE SILL OF LTTGAR. 



93 



a large fault-plane is well exposed in the bed of the stream. This, 

 with its fellows, has determined the course of the stream for some 

 distance. It is a fissure about a foot wide, filled with calcite and a 

 yellow flinty material. The rock on both sides of the major fault 

 belongs to the ultrabasic stratum. It is much crushed, splintered, 

 and traversed by numerous thin, anastomosing veins of calcite. 

 Sedimentary rock, however, is exposed on the inner side of the 

 bend, and consists of black shale upturned steeply at the fault. 

 High up in the bank on the west side of the stream, highly 

 analcitic teschenite is exposed, passing quickly into a dense black 

 contact-facies containing sparse flakes of biotite. At the outer 

 edge of the first right-angle bend above Bellow Bridge occurs 

 a black-and-white mottled rock belonging to the lower band of 

 teschenite. At the fault the continuous section of igneous rock 

 ends, and the stream cuts into the sedimentaries. Hence the lower 

 contact is not here visible. 



(3) Other Exposures. 



Between the Bellow and Glenmuir Waters, near their confluence, 

 rises a knoll of high ground. The Ayr-Muirkirk railway crosses 

 the Glenmuir Water here, and is carried through the knoll in a 

 shallow cutting. As one ascends from either stream, exposures 

 of the theralitic facies and the teschenite are encountered, capped 

 by a small outlier of the 'Millstone Grit' (see map & section, 

 fig. 3, p. 91). The railway-cutting shows nothing but decomposed 

 crumbling picrite. 



At the extreme north-eastern end of the sill, a mile and a half 

 north-east of Cronberry, a small exposure is seen in the Bellow 

 Water, here known as the Gass Water. At the south-western end 

 of the section a fine-grained marginal facies of pink teschenite 

 (doubtless the top of the sill) is observed, followed by grey theralite 

 and decomposed picrite towards the north-east. The lower teschenite 

 is not seen ; but, so far as it goes, the sequence here is identical with 

 that in the typical exposures. Another section is seen in the Lugar 

 Water at Logan Bridge, at the extreme south-western end of the 

 outcrop. Here the intrusion is quite thin, measuring not more than 

 15 or 20 feet in thickness, and is wedging out westwards among 

 the sandstones. Both contacts, bordered by hardened white sand- 

 stones, are seen. The marginal facies is a decomposed brownish 

 aphanitic rock showing a few flakes of biotite. The interior 

 consists of decomposed teschenite, but there is no trace of the 

 other facies. 



A mass of theralite probably connected with the Lugar sill, but 

 with a distinct dyke habit, crosses the Lugar Water in a north to 

 south direction, about 250 yards west of Logan Bridge. The 

 contacts are not seen ; but, on the northern bank of the river, the 

 rock forms a small knoll with vertical sides, and has the aspect of a ■ 

 dyke. In appearance and microscopic structure it is identical with, 

 the dominant phase of the theralite stratum in the Lugar sill. 



Q. J. G. S. No. 286. i 



