﻿88 



ME. G. W. TYRRELL ON THE 



[vol. lxxii, 



numerous veins of coarse-grained teschenite with abundant anal- 

 cite, similar to the rocks described below. 



The upper teschenite. — From a maximum depth of 12 feet 

 or so in tbe mass, the granularity of the rock increases very 

 rapidly towards the interior. The contact-facies passes into a 

 coarse-grained mottled rock, the prevailing tint of which is pink or 

 dark green, according as the felsic or mafic minerals dominate the 

 colour. In thin section the rock is seen to be a typical teschenite, 

 composed of essential plagioclase and analcite, titanaugite, and 

 ilmenite, with accessory orthoclase, olivine, barkevikite, and biotite. 

 The texture is evenly granular, although the felspars sometimes 

 tend to take on a lathy or columnar form. In another variety the 

 augite is conspicuous as long thin black prisms, ranging up to 

 2 inches in length. A third variety shows very abundant analcite 

 in large pink masses, which are frecmently spherical. The rock 

 seems to become richer in analcite as a greater depth in the sill is 

 attained. The thickness of the teschenite cannot be measured in 

 this section, as the contact with the underlying facies is not well 

 seen ; but it is usually between 15 and 20 feet. 



There are, however, considerable variations in the thickness of 

 this band. Along the western bank of the Glenmuir Water, above 

 the lugarite locality (see p. 91), indurated sandstones and contact- 

 basalts occur less than 20 feet above the picrite. Hence the 

 theralite and teschenite intervening between the picrite and the 

 upper contact must be attenuated, or one of them must be absent 

 in this part of the section. 



The theralite band. — Underbying the teschenite is a 'fine- 

 grained, almost aphanitic, grey rock, which, under the microscope, 

 is seen to have the composition of a theralite. The mafic minerals, 

 olivine and titanaugite, are dominant over the plagioclase and 

 nepheline. Barkevikite, biotite, and iron-ores are rather abundant 

 accessories, and there is often a small amount of analcite. This 

 rock forms a band perhaps 10 feet thick ; but in the Glenmuir 

 gorge its outcrop mainly occurs high up in a vertical cliff, although 

 it may also be examined in small exposures on the eastern side of 

 the stream, and in the river-bed when the water is low. In the 

 latter it may be easily distinguished by its smooth, polished, water- 

 worn surface, in contradistinction to the coarse-grained, pitted 

 surfaces of the overlying teschenite and the underlying picrite. 

 Further details of the theralite band are reserved for the de- 

 scription of the Bellow section, where it is much better exposed. 



The picrite and peridotite. — By a gradual diminution in 

 the amount of felspar and nepheline, the theralite passes into 

 coarse-grained ultrabasic rocks, composed mainly of olivine and 

 titanaugite, frequently with abundant barkevikite, some iron-ore, 

 biotite, plagioclase, and analcite. These rocks constitute the' major 

 portion of the sill, and occup} r the interior of the mass. The upper 



