278 MR. W. E. WATT ON THE GEOLOGY [June 1914, 



decomposed to a coarse dark ' gravel ' surrounding several hard, 

 more resistant, cores. In section these prove to consist of 

 basic plagioclase, diallage, and hypersthene — the two last-named 

 being often intergrown, — together with a little olivine, brown 

 hornblende, and magnetite. The olivine has become serpentinized, 

 while the felspar is slightly turbid. Very similar to this is the 

 norite-pegmatite traversing the basic intrusion in the quarry below 

 Broadland. Here a pleochroic, schillerized, purplish hypersthene 

 is the predominant ferromagnesian mineral, accompanied by brown 

 hornblende, biotite, and magnetite. The plagioclase is slightly 

 altered, but the light-coloured dyke has largely decomposed. 



In the cutting of Rothiemay Station, several dykes are exposed, 

 not as a rule in a very fresh state. The least altered belongs to 

 the norite group, with a pleochroic hypersthene and a sub-ophitic 

 texture. In some cases olivine is developed, and the reaction-rims 

 surrounding this mineral have attained such a size as to destroy 

 the surrounding plagioclase — a destruction hastened by the 

 development of prehnite. The rhombic pyroxene may be nearly 

 colourless and very slightly pleochroic — an enstatite. In places 

 it surrounds the olivine, and the actinolite corona is developed 

 between the enstatite and the felspar. In this case a secondary 

 origin can be ascribed only to the actinolite. Chromite is also 

 present. 



A dyke of a bright green colour is found to consist mainly of 

 serpentine and scales of a chloritic mineral. Olivine-cores occur 

 in the serpentine, while, separable from the scales of chlorite by 

 their higher refractive index, occur laths of anthophyllite and 

 actinolite. The chlorite polarizes in pale greys and yellows of the 

 first order, and shows beautiful polysynthetic twinning — a type 

 not uncommon as a secondary mineral in dykes. 1 



In the extreme east of the cutting occurs a fine-grained dyke, 

 composed mainly of labradorite and secondary green hornblende. 

 The margin of this dyke is schistose and much decomposed, but 

 noteworthy for the veins of prehnite that traverse it. In the 

 development of prehnite' and occasional schistosity, these dykes 

 recall the features of many of those of the Lizard area. 3 



Acid veins from the intrusions of Carvichen, Avochie, and 

 Ordiquhill are very common, and call for little comment. They 

 consist of quartz, orthoclase, andesine or labradorite, and biotite. 

 As a rule, the norite becomes altered to a rock containing basic 

 plagioclase and green hornblende, as at Haddoch and the cutting 

 near Boghead of Kinnoir. 



1 ' Geology & Structure of the N.W. Highlands of Scotland ' Mem. Geol. 

 Surv. 1907, p. 209, &c. ; ' Geology of Ben Wyvis, &c.' Mem. Geol. Surv. Scot 

 1912, Explan. Sheet 93, pp. 84, 117. 



3 Mem. Geol. Surv. 1912, Explan. Sheet 359, p. 116. 



