Vol. 70.] OF THE COfXTBY AEOUXD HTTBTTLT. 273 



This fine-grained sub-ophitic rock is composed of labradorite and 

 pyroxene, both rhombic and monoclinic, together with some free 

 quartz. The pyroxene may give rise to a secondary green hornblende, 

 which, in the specimens from a neighbouring stream — the Stripe 

 of Clashancape, — is the dominant ferromagnesian mineral. By 

 some, this is considered to represent the original rock now in the 

 condition of serpentine, 1 but it is really an unfoliated dyke-rock 

 cutting the earlier foliated serpentine. It is thus comparable, not 

 only in mineral composition, but also in age, with the unfoliated 

 intrusions in the Huntly area. 



(b) The gabbro-picrite group. — The olivine-gabbro, the 

 troctolite, and the picrite, which occupy the western portion of 

 the area, are simply lithological variations of the noritic magma. 

 Of the first type, the ojlivine-gabbro, the essential minerals are 

 felspar, olivine, and pyroxene, and to these brown hornblende, 

 biotite, magnetite, and pyrite are to be added. Of secondary 

 origin are serpentine, anthophyllite, and actinolite. 



As before, the felspar is rather basic, but the flakes examined 

 from different gabbros show that the composition has a considerable 

 range. Occasionally the mean refractive index reaches 1*570, but 

 more usually the felspar is a labradorite with a refractive index of 

 1'560 to 1*564. A slight turbiditv mav be noticed, and the more 

 basic types show some schillerization. 



Of the two varieties of monoclinic pyroxene, diallage predominates 

 in its characteristic form, its brown masses and grains being in 

 marked contrast with the pale greenish ophitic variety. Barely, 

 the latter predominates. The rhombic pyroxenes are represented 

 by pleochroic hypersthene. which, when present, is often associated 

 with the diallage. Its comparative scarcity contrasts strongly 

 with its abundance in the norite. 



From the serpentinized olivine, the usual fissures traverse the 

 felspars, while the resulting serpentine, yellowish or greenish in 

 colour, is (with the residual olivine) strongly charged with magnetite 

 dust. Between olivine and felspar are developed reaction-iims of 

 two or three minerals — anthophyllite and actinolite, with or 

 without brown hornblende. The anthophyllite, distinguished by 

 its straight extinction from the actinolite, and by its cross-cleavage 

 from a rhombic pyroxene, occurs next the olivine. Outside this is 

 sometimes developed a layer of brown hornblende, which, however, 

 ma}' be absent. Outside again, and next the felspar, pale-green 

 pleochroic actinolite occurs. These rims may be of considerable 

 size: one had needles of anthophyllite 5*8 mm. long, followed by 

 needles of actinolite of a length of 5 '(5 mm. 



The brown hornblende is usually charged with magnetite dust ; 

 occasionally it forms a shell to the pyroxene, but it is much I 

 common than in the norite series. Biotite is rare ; occasionally 

 it forms a ring round magnetite, as do actinolite and brown 



1 Mem. Geol. Surv. Scot. 1S90, Explan. Sheet 7G. p. 26. 



