﻿part 2] 



PICKJLTE-TESCHENITE SILL OE LTJGAE. 



89 



part of the ultrabasic stratum is rich in titanaugite and barkevikite, 

 as well as olivine, and usually contains some plagioclase and anal- 

 cite. This is a true picrite in the original sense of Tschermak, 

 who applied the term to a melanocratic facies of the Moravian 

 teschenite. In the remainder, however, plagioclase and analcite 

 completely disappear, and olivine becomes the dominant mineral. 

 This rock is a hornblende-periclotite, in which the alkalic tendency 

 is still recognizable by the unusual amount of alkalies contained in 

 the bisilicates. A tongue-like extension of the ultrabasic outcrop 

 occupies the bed of the Grlenmuir Water for some distance north- 

 west of the railway- viaduct, the overlying facies forming the 

 cliffs at this point. Good waterworn surfaces can be examined 

 here when .the river is low. The rock is remarkably fresh, and 

 shows sharp and irregular variations in granularity. East of the 

 railway-bridge, the ultrabasic stratum forms nearly the whole of a 

 great cliff rising vertically to a height of 90 feet from the north 

 side of the stream. Here, where it is exposed to the weather and 

 is not being continually scoured by the water, it is much decom- 

 posed, and weathers to a loose crumbling mass, with spheroidal 

 lumps of harder and fresher rock in places. The north-westerly 

 inclination of the intrusion is well shown in ijhe face of the 

 cliff by the inclination of the joint-planes, but principally by the 

 contrast between the jointing of the ultrabasic rock and the over- 

 lying theralitic facies. The former is traversed by a few large 

 irregular joints ; but the latter by many, both vertical and hori- 

 zontal, causing a rude columnar structure. While the transition 

 between the two is not sharp, it is sufficiently well marked to show 

 the general dip of the "intrusion towards the north-west. The 

 ultrabasic rock is traversed by a few thin veins of a fine-grained 

 felspathic rock (teschenite-aplite). 



The lower teschenite. — A blank of about 30 feet in the 

 section separates the last exposure of the ultrabasic stratum from 

 the underlying facies, which consists of teschenite made up, for the 

 greater part, of the variety containing numerous blade-like or 

 columnar augites. This is exposed in a thick bar across the 

 stream near the second right-angle turn above the railway-viaduct : 

 its thickness is estimated at about 20 feet. It is important to 

 notice that no theralite intervenes here between the picrite and 

 the teschenite. There is little reason to suppose that it is hidden 

 by the blank in the section, for the theralite is the most resistant 

 rock in the complex to ordinary atmospheric weathering. 



The lower contact. — The granularity of the lower teschenite 

 declines rapidly, until it passes into the ordinary contact-basalt. 

 The latter shows banding precisely similar to that of the upper 

 contact, and is also traversed by veins of coarse pink teschenite. 

 The two most prominent varieties involved in the banding, which is 

 frequently much confused and contorted, are a fine-grained pink 

 teschenitic rock and a dense black or grey basalt, the two rocks 



