The Petrology of the Arnage District in Aberdeenshire: A Study of Assimilation

Abstract

In the Buchan, Formartine, and Strathbogie districts of Aberdeenshire, the plutonic rocks of lounger 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. R. Watt 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 Prof. A. Lacroix and from Snowbank Lake (Minnesota) by Prof. A. N. Winchell. After the War it was my good fortune to revise for the Geological Survey the country around Huntly and Banff (Sheets 86 & 96 of the 1-inch Geological Survey Map of Scotland), and 1 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. 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 Dr. A. Harker was considered to have a more specialized meaning, since it has come to denote rocks formed either 'by the mixture of two magmas, or