448 Me. h. h. bead on the [vol. lxxix, 



II. Previous Wobk. 

 Before this investigation no modern work had been done on the 

 Arnage Mass. This mass forms part of the ' diorite ' of J. S. 

 Grant Wilson, 1 as described in the Survey Memoir of 1886. 

 The excellent norites of Arnage have been mentioned by H. llosen- 

 busch 3 and briefly described and figured by Dr. A. Harker. 3 

 Sir Jethro Teall l foreshadowed the main feature of the Arnage 

 Mass, when in 1896 he discussed certain cordierite-bearing rocks 

 of Scotland. Of one specimen from Little Arnage he said : — 

 'it is evidently a compound rock due to the superposition of 

 igneous upon metamorphic material' (op. cit. p. 37). 



III. The CouNTBr-RocKs. 

 The rocks into which the Arnage Mass is intruded, and which 

 supply the material for contamination of the original magma, 

 belong to two main groups : — 



(i) A series of andalusite-cordierite-schists and felspathic quartzites, 

 with quite subordinate impure limestone-bands, occurring west, 

 north, and north-east of the igneous and contaminated rocks. 

 (See fig. 2, p. 452.) 



(ii) A series of biotite-gneisses and subordinate hornblende-schists, 

 occurring south-east of the igneous and contaminated rocks. 

 (See fig. 2, p. 452.) 



The first group, occurring north of a line drawn from Ardlethen 

 to Berefold, may be considered as made up of a western argil- 

 laceous and an eastern quartzose sub-group. On the west the 

 dominant rock-type is an andalusite-cordierite-schist with sub- 

 ordinate quartzite, while on the east quartzites with subordinate 

 andalusite-schists occur. The western andalusite-schists are the 

 north-eastern extension of a series of andalusite-schists well seen 

 in the Ythan Valley between Fy vie and Methlick ; these have been 

 called the Fyvie Schists, 5 and lithologicaily they are exactly 

 similar to another group of andalusite-schists — those of Boyndie 

 Bay 6 — on the Banffshire coast. It is extremely probable that 

 these two groups are the same, and occur on opposite sides of a 

 central synclinal area of Macduff Slates. 7 



The eastern quartzose sub-group was separated out by J. S. 

 Grant Wilson, 8 and to it was assigned a stratigraphical status 



1 ' Explanation of Sheet 87 ' Mem. Geol. Surv. Scot. 1886, p. 17. 



12 ' Mikroskopische Physiographie ' 4th ed. vol. ii, pt. 1 (1907) p. 349. 



3 ' Petrology for Students' 5th ed. (1919) pp. 77-78 & fig. 23 B. 



4 'Explanation of Sheet 75 ' Mem. Geol. Surv. Scot. 1896, pp. 37, 38. 



5 H. H. Bead, ' The Geology of the Country round Banff, Huntly, & Turriff ' 

 (Explan. of Sheets 86 & 96) Mem. Geol. Surv. Scot. 1923, Chap. iv. 



fi Id. ibid, and ' The Banffshire Coast-Section of the Highland Schists ', 

 App I to ' Summary of Progress of the Geological Survey for 1920' Mem. 

 Geol. Surv. 1921, p. 76. 



7 Id. 'The Geology of the Country round Banff, Huntly, & Turriff' (Explan. 

 of Sheets 86 & 96) Mem. Geol. Surv. Scot. 1923, Chap. iv. 



8 ' Explanation of Sheet 87 ' Mem. Geol. Surv. Scot. 1886, pp. 9-11. Map, 

 Sheet 87. 



