446 Me. ir. it. read ox the [vol. lxxix, 



17. The Petrology of the Arnage District in Aberdeen- 

 shire : A Study of Assimilation. By Herbert Harold 

 Bead, M.Sc, A.B.C.S., F.G.S. (Bead March 14th, 1923.) 



Contents. 



rage 

 I. Introduction 446 



II. Previous Work 448 



III. The Country-Rocks 448 



IV. The Norite Sheet 453 



V. The Contaminated Rocks 457 



VI. The Xenoliths and Hornfelses 473 



VII. The Contamination Process 479 



VIII. Conclusion , 484 



I. INTRODUCTION. 



In the Buchan, Formartine, and Strathbogie districts of Aberdeen- 

 shire, the plutonic rocks of ' Younger Granite ' age include half-a- 

 dozen large gabbro-masses the approximate positions of which are 

 shown in fig. 1 (p. 447). These gabbros are later than the regional 

 folding, but antedate the local Middle Old Bed Sandstone. They 

 possibly form one huge sill. 



In 1914, Mr. W. B. Watt 1 described the geology of the 

 southern part of the Huntly Mass, and announced the discovery of 

 gabbros modified by the incorporation of sedimentary material. 

 These endomorphic gabbro-derivatives were shown to be similar in 

 character and origin to rocks recorded from Le Pallet (France) 

 by Brof. A. Lacroix, 3 and from Snowbank Lake (Minnesota) by 

 Prof. A. N. Winchell. 3 After the War it was my good fortune 

 to revise for the Geological Survey the country around Huntly and 

 Banff 4 (Sheets 86 & 96 of the 1-inch Geological Survey Map of 

 Scotland), and I was then able to investigate the distribution, 

 nature, and origin (in the Huntly Mass) of the abnormal rock- 

 types discovered by Mr. Watt. Later, similar rocks were found 

 in the Insch Mass. 5 



As their importance in the field increased, it was felt desirable 

 to have some handy term for rocks resulting from the incorporation 

 of sedimentary material in a magma, and, in 1921, the name 

 contaminated rock was suggested. The term hybrid of 



1 ' The Geology of the Country around Huntly (Aberdeenshire) ' Q. J. G. S. 

 vol. lxx, p. 266. 



2 ' Le Gabbro du Pallet & ses Modifications ' Bull. Carte Geol. France, 

 vol. x (1898-99) pp. 341-96. 



3 ' Mineralogical & Petrographic Study of the Gabbroid Rocks of Minne- 

 sota, &c.' Amer. Geol. vol. xxvi (1900) pp. 294 et seqq. 



4 H. H. Read, 'The Geology of the Country round Banff, Huntly, & Turriff' 

 (Explan. of Sheets 86 & 96) Mem. Geol. Surv. Scot. 1923. Interim reports in 

 ' Summaries of Progress ' for 1918, 1919, 1920. 



5 Id. ' The Contaminated Gabbro of Easter Saphock, near Old Meldrum in 

 Aberdeenshire' Geol. Mag. vol. lviii (1921) pp. 177-83. 



