﻿102 



ME. a. W. TYRRELL ON THE 



[vol. lxxii, 



notably in the Cathcart type from the lower band of teschenite. 

 It is altered to a highly-polarizing, scaly, micaceous substance. 

 Apatite is abundant, and is enclosed in all the other constituents 

 of the rocks. 



The above description covers rocks belonging to the Glasgow, 

 Galston, and Cathcart types. In addition to these are one or 

 two abnormal rocks which may be regarded as highly felsic and 

 mafic varieties of the teschenites. The former occurs as a phase 

 of the variable analcite-rich layer which intervenes between the 

 teschenite proper and the underlying theralite. The orthoclase is 

 approximately equal in amount to the plagioclase, and the rock is 

 clearly leucocratic. Teschenitic rocks comparatively rich in ortho- 

 clase are mentioned in Dr. Flett's account 1 of the teschenites of 

 the Edinburgh district. These rocks might be called analcite- 

 monzonites, analogous with the nepheline and leucite-monzonites. 



The melanocratic variety also occurs near the junction of the 

 teschenite and the theralite, where the analcite-band is absent. Its 

 mineral composition is denned in Table I, col. v (p. 103). It shows 

 a decided predominance of mafic minerals, and the whole aspect of 

 the rock is ultrabasic. Olivine becomes an essential constituent; 

 augite with hornblende borders is abundant, as well as biotite. 

 Ilmenite, for some unexplained reason, dwindles in this as in all 

 the more basic and ultrabasic rocks of the series. Analcite has 

 decreased considerably in quantity ; and, concomitantly, the felspar 

 generally is very fresh. The latter was one of the last minerals to 

 crystallize, fills up the irregular interspaces between the mafic 

 constituents, and shows radiating cracks due to the expansion of 

 serpentinized olivine. 



The position of this rock is rather perplexing. It may be inter- 

 preted merely as a local, very basic, schlieren ; or as due to a small 

 and localized gravity- stratification in the upper teschenite. Its 

 mineral composition shows that it cannot be regarded as a transi- 

 tion -facies from the teschenite to the theralite. The theralites are 

 entirely different, both mineralogically and texturally. A melano- 

 cratic teschenite has been described by Prof. W. ,T. Sollas from New 

 Zealand. 2 That rock, however, is rich in titaniferous magnetite 

 and poor in olivine, thereby differing in these respects from the 

 Lugar rock. 



Quantitative Mineral Composition of the Teschenites. 



These rocks lend themselves well to micrometric analysis by the 

 Rosiwal method. The chemical compositions calculated from the 

 mineral analyses agree Avell with the actual chemical composition, 

 as determined by the usual methods of analysis, with the general 

 exception of potash (see Table II, p. 101). This is due to the 



1 ' The Geology of the Neighbourhood of Edinburgh ' Mem. Geol. Surv. 

 Scotland, 1910, p. 294. 



3 ' Rocks of the Cape Colville Peninsula, Auckland (New Zealand) ' vol. ii 

 (1906) p. 156. 



