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



IY. — The Two Magmas of Strathbogie and Lower Banffshire. 

 By H. H. Read, H.M. Geological Survey, Scotland. 



THE field-work upon which this communication is based was 

 carried out in 1917 and 1918, in preparation for the Geological 

 Survey map of Sheet 86 (Huntly). 



In the Strathbogie district of Aberdeenshire and in Lower 

 Banffshire : two series of igneous rocks have a wide distribution. 

 The earlier of these series was intruded prior to the movements 

 which caused the foliation and schistosity of the Dalradian sediments, 

 the later after these movements had ceased. The object of this 

 paper is to detail the diversity of original rock types constituting 

 the Older Series, to delineate their magmatic sequence, and to 

 compare them, both in order of intrusion and in petrographic 

 characters, with the similarly diversified Younger Series. 



1. The Older Series. 



Igneous rocks of prefoliation age have a wide distribution in this 

 district, as will be seen from the Sketch-map. These Older rocks 

 are mostly altered to serpentine, tremolitic serpentine, epidiorite, 

 amphibolite, hornblende-schist, and augen-gneiss, but it lias been 

 found possible to trace back the serpentines to pyroxene and olivine 

 rocks, the epidiorite, amphibolite, and hornblende-schist to gabbro 

 and enstatite gabbro, and the augen-gneiss to granite. The most 

 important locality of the Older Series from this point of view is 

 that of Portsoy, Banffshire. The rocks of this locality have been 

 described many years ago by Jameson, 2 Cunningham, 3 and Heddle. 4 



The non-fel spathic ultrabasic members of the Older Series are 

 usually in the condition of serpentine, occasionally well foliated but 

 more often massive. Throughout the district, however, rocks 

 composed wholly of monoclinic pyroxene are strongly developed and 

 can be found in all stages of transformation to serpentinous and 

 tremolitic derivatives. Such anchi-monomineralic rocks form 

 independent intrusions, and since they are cut by epidiorite may 

 be considered as the earliest manifestation of the Older igneous 

 activity. Type localities are at Portsoy, at Whitehill west of 

 Kothieinay Station, and on both flanks of Evron Hill. The 

 pyroxenite is a coarse massive greenish-grey non-foliated rock 

 which in thin slice is seen to consist of large interlocking crystal 

 plates of a pale-coloured augite. No felspar has been seen in the 

 rock. Occasionally olivine in small grains is scattered sporadically 

 throughout the rock and may have been locally predominant. 

 Enstatite and hornblende, probably primary, also occur at certain 

 localities. There is never any banding of the constituents, 

 collection of olivine in layers or fluxional arrangements of the 



J Further notes on these districts will appear in the Summary of Progress 

 of the Geological Survey of Great Britain for 1918. 



2 R. Jameson, Mineralogy of the Scottish Isles, etc., 1800, vol. ii, p. 270etseq. 



3 Hay Cunningham, " Geognostic Account of Banffshire" : Trans. High, 

 and Agric. Soc. Scot., ser. II, vol. viii, p. 447, 1842. 



4 M. F. Heddle, " Chapters on the Mineralogy of Scotland " : Trans. Boy. 

 Soc. Edin., vols, xxvii-xxix, 1876-80. 



