﻿92 



MR. Q. W. TTKEELL ON THE 



[vol. lxxii, 



above the confluence with the Glemimir Water, the upper contact 

 of the sill with a soft, yellow, crumbling sandstone is well seen. 

 The contact-facies is a brownish aphanitic rock, with some sporadic 

 flakes of biotite. It is obviously much more decomposed than the 

 corresponding rock in the Grlenmuir section. The flow-banding 

 also is not at all prominent. 



The contact-facies passes clown quickly first into a fine-grained 

 pink and green mottled teschenite, and then into a coarser rock 

 with a conspicuous development of large columnar augites. 

 Although this is the dominant facies there are a few bands of fine- 

 grained material, which are doubtless due to the flow of a slightly 

 heterogeneous magma. Still lower down the rock becomes distinctly 

 more felsic, and shows abundant pink and white spots of analcite. 

 In this variety the augite-crystals are not columnar. 



This analcitic facies is in sharp contact with the dark, fine- 

 grained, theralitic stratum beneath. The stream has cleared this 

 contact rather thoroughly, owing to the differential resistance of the 

 rocks to erosion. It slides down a polished waterworn surface of 

 theralite, which is bordered on one side by a low cliff of the coarser " 

 and softer teschenite. Hence the contact can be particularly well 

 examined. The theralite is here seen to be shot through or per- 

 meated with irregular masses, patches, nests, strings, and anasto- 

 mosing veins of a pinkish, coarse-grained, felsic rock, resembling 

 the overlying teschenite. These patches and veins do not appear 

 to be intrusive. They have no sharply-defined contacts with the 

 theralitic facies, but the minerals interlock at the margins, welding 

 the two rocks into an intimate union. There is no sign of chilling 

 at the margin of the patches, the normal granularity being main- 

 tained up to their edges. There can hardly be any doubt that 

 the theralitic rock and these felsic veins and patches crystallized 

 practically at the same time. The theralitic rock immediately 

 beneath the band of analcitic teschenite is full of these veins and 

 patches, which are also found at lower horizons (although in greatly 

 diminished quantity), until they finally disappear long before the 

 picrite is reached. The overlying analcitic teschenite is not a 

 continuous band, but is developed irregularly, sometimes dying 

 out altogether, in such wise that the augitic facies comes into 

 contact with the theralite. 



By the gradual disappearance of the felsic minerals, the theralite 

 passes into picrite, although at one or two places lugarite appears to 

 separate the two rocks. Owing to the conformation of the Bellow 

 gorge, the ultrabasic outcrop has a tongue-shaped lobe directed 

 westwards, which occupies the bed of the stream, while the over- 

 lying facies form the sides of the valley. The ultrabasic rocks have 

 precisely the same characters as in the Glenmuir section. At and 

 beyond Bellow Bridge the outcrop Avidens, but the overlying thera- 

 litic and teschenitic facies ma,j still be traced in the steep wooded 

 slopes west of the bridge. At the first right-angle bend of the 

 stream above Bellow Bridge the picrite is intersected by many 

 small crush-planes filled with ' beefy ' calcite. At the second bend 



