﻿part 2] 



PICEITE-TESCIIENITE SILL OF LU.GAB. 



91 



The lugarite. — On the west bank of the river, under the 

 railway-bridge and a little to the north side of it, the cliff exhibits 

 a sharp horizontal junction between decomposed picrite and a 

 peculiar rock overlying it. The latter shows a greyish or greenish 

 aphanitic ground-mass, crowded with both stout and slender black 

 prisms. Sometimes small rectangular whitish felspars are seen 

 also. In thin section the ground-mass may be identified as analcite, 

 crowded with needles of apatite, and containing some nepheline. 

 In it are scattered numerous perfectly euhedral prisms of barke- 

 vikite, with subordinate titanaugite and plagioclase, also extremely 

 well formed. This rock forms a stratiform mass about 4 feet 

 thick overlain by the theralitic facies. The junction is quite sharp, 

 as the two very distinct rocks can be got within an inch of each 

 other : but the line of demarcation is, in general, not marked by 

 any definite feature on the face of the cliff. The two rocks are 

 intimately welded without any appearance of passage. The long 

 prisms of barkevikite, however, project from the lugarite into the 

 theralite. At places the junction is marked by a thin, fine, 

 horizontal joint-plane, or by a thin layer of slightly decomposed 

 rock. The contact with the picrite is doubtless of the same nature, 

 but the latter is so decomposed that the junction is quite obscured. 

 Some veins of lugarite penetrating the picrite supply further 

 evidence on this point. These veins or small dykes range up to 

 4 inches in width, and are largely composed of analcite and 

 barkevikite prisms, which are frequently arranged in stellate 

 groups. The contact of these veins with the picrite is an intimate 

 welding, and the barkevikite crystals project from the sides of the 

 veins into the enclosing rock. 



The lugarite is also to be found in the Bellow section in the 

 same relative position, but rather poorly exposed. The Glenmuir 

 exposure is only accessible when the stream is low ; but south 

 of the viaduct occur numerous fallen blocks from an inaccessible 

 exposure in the face of the cliff. In the other direction the dip of 

 the intrusion carries the lugarite down to the water's edge, where 

 it is lost. 



All the available evidence goes to show that this remarkable rock 

 is intrusive in the picrite. The veins of similar material traversing 

 the picrite transgress sharply the different varieties of the ultra- 

 basic rock. There can be no doubt that these veins proceed from 

 the main mass of lugarite, although the junctions have not actually 

 been demonstrated. While the rock is clearly intrusive, it is but 

 slightly posterior to the main phenomenon of intrusion, for the 

 intimate welding and absence of chilled rock at the margins indi- 

 cates that the ultrabasic rock was still very hot at the time of 

 intrusion. 



(2) The Bellow Section. 



Although not so complete, this section supplies many details 

 which are missing or obscure in the Glenmuir section. Just 



