part 4] PETROLOGY OF THE ARNAGE DISTRICT. 479 



VII. The Contamination Process. 



For the purposes of this paper I shall regard the Arnage norite 

 as the initial magma of the Arnage district, although I believe it 

 likely that this view will be found to require some modification as 

 research in contaminated rocks progresses. 



We may consider the results which followed from the intrusion 

 of a sheet-like mass in the Arnage ground. At the top of the 

 sheet was formed a xenolithic zone some hundreds of feet thick 

 (see section, fig. 2, p. 452) ; this zone we now see as the contami- 

 nated zone lying above the norite sheet. 



The matrix of the xenolithic zone is what has here been 

 described as contaminated rock. Contaminated rocks derived 

 from gabbros necessarily are extremely variable ; but there are 

 certain striking regularities about their compositions which are of 

 great importance. In Table V, p. 480, are enumerated the probable 

 initial magmas and their contaminated derivatives for the Arnage, 

 Insch, 1 and Huntly 3 Masses in the North-East of Scotland, for 

 Le Pallet 3 in France, and for Minnesota 4 : that is, for the five 

 described cases of contaminated gabbros. There are available at 

 present three analyses of contaminated rocks of Arnage, one of 

 Insch and two of Huntly; Prof. Lacroix gives six analyses of 

 contaminated gabbros from Le Pallet which are averaged in 

 Analysis T> v Table V, while the average of two analyses of his 

 initial magma is represented by Analysis D ; Prof. Winchell 

 supplies one analysis of the Snowbank Lake contaminated 

 gabbro, and his initial gabbro is taken to be the olivine-gabbro 

 also analysed by him. There are available, therefore, thirteen 

 analyses of contaminated gabbros. 



The chemical differences between the initial magmas and the 

 contaminated derivatives may be deduced from Table V. The 

 chief oxides are dealt with here. Silica is mainly increased in the 

 contaminated magmas ; alumina (with one exception) increases, 

 and sometimes markedly ; there is a general rise in ferrous oxide, 

 but of no great magnitude in any case ; lime is always much less ; 

 magnesia, with one exception, is much less too ; potash in the 

 majority of cases is increased ; soda is rather indefinite, but seems 

 upon the whole to increase ; soda is always greater than potash in 

 the initial magmas, but the disparity is either less marked, or 

 potash is in excess of soda in the contaminated magmas. 



Summarizing these observations, we find that in contamination 

 the gabbro magmas become richer in alumina and potash, and 

 poorer in lime and magnesia ; iron-oxides and soda appear to play 

 no constant part. 



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

 (Explanation of Sheets 86 & 96) Mem. Geol. Surv. Scot. 1923, Chap. VII. 



2 Id. ibid. ; and W. E. Watt, ' Geology of the Country around Huntly 

 (Aberdeenshire) ' Q. J. G. S. vol. lxx (1914) p. 266. 



3 A. Lacroix, ' Le Gabbro du Pallet & ses Modifications ' Bull. Carte Gcol. 

 France, vol. x (1898-99) p. 341. 



4 A. N. Winchell, ' Mineralogical & Petrographic Study of the Gabbroid 

 Rocks of Minnesota, &c.' Amer. Geologist, vol. xxvi (1900) p. 151. 



