H. H. Read — The Two Magmas of Strathbogie. 369 



foliation of the country. The younger igneous rocks of the 

 "Wardhouse district will not be considered here. That part of the 

 Huntly Mass which occupies the district around Htintly has been 

 described in detail by W. B. Watt, 1 but the whole of the area shown 

 in the Sketch-map lias been mapped by the present writer. The 

 rocks of the Huntly Mass, with its isolated granite bosses of 

 Aberehirder, Ord, and Longmanhill (4 miles E.S.E. of Banff ), form 

 a lengthy sequence, here appended in order of intrusion: — 



1. Picrite-Norite Set '(Picrite, Olivine-gabbro, Troctolite, Norite). 



2. Diorite of Gibstone. 



3. Monzonite and Diorite (Central Intrusion of TV. E. Watt). 



4. Granite. 



5. Pegmatites with tourmaline. 



The picrite-norite set are well seen around Huntly and extend 

 thence northwards to within two miles of Portsoy. The picrite 

 forms a narrow border on the western edge of the Huntly Mass in 

 the Huntly district. It is a black rock with conspicuous lustre 

 mottling and in thin slice is rather variable. Occasionally it 

 approximates to a peridotite, but usually carries common ophitic 

 pyroxene with relatively scarce basic plagioclase. Brown horn- 

 blende, biotite, and hypersthene occur in small amount. The olivines 

 are slightly serpentinized. No trace of cataclastic structure is found. 

 The olivine-gabbro occurs east of the picrite and forms a coarse 

 very fresh rock with labradorite, olivine, and pyroxene, with sub- 

 ordinate brown hornblende and biotite. The troctolitic type occurs 

 mainly east of the olivine gabbro and is a characteristic olivine- 

 labradorite rock with beautiful fluxional arrangements of the felspars 

 .and with banding of more basic types of peridotite, etc. These 

 fluxion structures are parallel to the margin of the mass, as is usual 

 amongst fluxional gabbros. 2 Such structures are best explained for 

 the Huntly Mass as due to movement during the intrusion of a some- 

 what past} 7 and not strictly homogeneous magma. Troctolite also 

 occurs at the western edge of the Huntly Mass west of Knock Station. 

 Beautiful reaction rims of anthophyllite and actinolite are developed 

 between the olivine and felspar of the troctolitic and olivine gabbros. 3 

 The predominant body of the picrite-norite set is, however, the 

 norite. This type forms nine-tenths of the Huntly Mass. It is 

 usually a fine-grained bluish rock, consisting of labradorite-bytownite 

 and hypersthene with widespread augite, hornblende, biotite, and 

 olivine. Structures are variable, being ophitic, granular, or granitic. 

 Occasionally the norite lacks its hypersthene and then forms a very 

 coarse gabbro. 



The members of the series — picrite, olivine-gabbro, troctolite, 

 norite — follow in order from west to east, and at first glance this 

 arrangement seems to point to a direct gravitative differentiation of 

 a gabbro magma in the position in which it is now found. But no 



1 W. K. Watt, "Geology of District around Huntly (Aberdeenshire)": 

 Q.J.G.S., vol. Ixx, pp. 266-93, 1914. 



2 F. F. Grout, "Internal Structures of Igneous Rocks": Journ. Geol., 

 vol. xxvi, p. 439, 1918. 



3 W. Pv. Watt, loc. cit., pi. xxxviii, fig. 2. 



DECADE VI. — VOL. VI. — NO. VIII. 24 



