458 ME. H. H. READ ON THE [vol. lxxix, 



shown in fig. 2 have heen plotted where no igneous material was 

 discernible in these ; but there is a gradual passage from country- 

 rock to contaminated rock, and from this to norite. No clearer 

 example of assimilation could be desired. 



Since these contaminated rocks are extraordinarily prone to rapid 

 variation, it is a matter of some difficulty to attempt their de- 

 scription, hut certain broad types and distributions can be recognized. 

 These are : — 



(A) A type which in composition approaches that of the norite ; this type 

 occurs in small patches kneaded in among the more contaminated 

 types, and may be called the Gabbro Type. 



(JB) The most widespread type, occurring typically north of a line drawn 

 from Ardlethen to Cookston (see fig. 2, p. 452), and consisting of 

 varying associations of quartz, cordierite, plagioclase, alkali-felspar, 

 and biotite, with rarer garnet, spinel, and tourmaline. This type 

 represents the result of the modification of the initial magma by 

 quartzose and argillaceous sediments of the Fyvie Series, and biotite- 

 schists of the Ellon Series. It will be here called the Arnage Type. 



(C) A dioritic type seen between the Little Arnage kernel and the River 



Ythan, and produced by the contamination of the initial magma with 

 hornblende- schists of the Ellon Series ; this type, from its splendid 

 exposures on the Kinharrachie estate, may be called the Kinhar- 

 rachie Type. 



(D) The rocks occurring south of the Ythan around Ardlethen are dealt with 



separately in this description, since here (associated with the other 

 types) is the best development of the granitic end-product of contami- 

 nation. This granitic end-product may be called the Ardlethen 

 Type : its importance from the petrogenetic standpoint needs no 

 emphasis. 



These four types, with their distribution and petrography, will 

 now be discussed in order. 



(A) The Gabbro Type of Contaminated Rock. 



These rocks appear to provide transitions between the norite and 

 the chief contaminated rock, that of Arnage Type. They occur 

 away from the kernels, as small patches intimately associated with 

 the contaminated rocks into which they pass. As an example, Ave 

 may take a small knob situated east of the Ebrie Burn, 500 yards 

 south of Nether Mill, where, from a rock-face only a few square 

 yards in area, there can be collected homogeneous gabbro types and 

 variable contaminated rocks crowded with xenoliths. Other good 

 localities for these little-modified types are in the wood east of 

 Waulkmill of Savoch, or at Mill of Elrick at the northern end of 

 the Arnage Mass. 



In itself, this gabbro type is variable, in sharp contrast with the 

 strictly uniform norite. Two chief varieties are noted : the first of 

 these can be seen in the wood east of Waulkmill of Savoch, or at 

 the Ebrie side 300 yards south-east of Mains of Drumwhindle, and 

 is a coarse grey rock showing large plates of biotite. In slice it is 

 found to be a quartz-gabbro made up of diallage, hypersthene, acid 

 labradorite, and quartz. The quartz is much more abundant than 



