182 H. H. Read—Contaminated. Gabbro in Aberdeenshire. 
whole Huntly Mass took place along its base before an ultrabasic 
bottom layer had time to collect. 
It must be admitted that of these three possibilities the last is the 
most attractive, and would be readily adopted if one was sure that 
any kind of differentiation in place had occurred in the Huntly Mass.+ 
On this point extremely conflicting evidence may be found if the 
whole of the mass is considered. As Watt remarked, it is unfortunate 
that these rocks of Cuternach occur in cultivated ground. No 
advantage can be obtained at present by dogmatic statements. 
The remainder of the occurrences of contaminated norites are 
fortunately free from doubt. The original purely igneous rock is 
the great norite ? of the Huntly Mass. This fine-grained bluish rock 
consists of ophitic, granular, or granitic associations of bytownite 
(Watt), hypersthene, augite, olivine, hornblende, biotite, magnetite, 
apatite, and zircon. 
A summary of the study of the contaminated norites may be given. 
These rocks are extremely variable in texture and composition, and 
contain partly digested hornfels chunks. The minerals observed in 
the contaminated norites are :— 
Cordierite, garnet, hypersthene, labradorite, andesine, orthoclase, 
biotite, spinel, sillimanite, magnetite, quartz, rutile, zircon. 
This assemblage is like that already given for the Saphook con- 
taminated gabbros. In the field, in hand specimen, and in slice, 
the contaminated gabbros of both the Huntly and Insch Masses are 
thoroughly alike. 
FoREIGN OCCURRENCES OF CONTAMINATED GABBROS. 
Professor Lacroix ? described the occurrence in the quarry of 
Les Prinaux at Le Pallet, Loire Inférieure, of xenoliths of mica- 
schist surrounded by a layer of cordierite-norite separating them 
from normal olivine-gabbro. Lacroix deduced that both norite and 
cordierite-norite had resulted from the incorporation of sedimentary 
material of suitable composition in the olivine-gabbro magma. At 
present I am not prepared to admit a similar origin for the norites 
of North-East Scotland. I am, however, in entire agreement with 
Lacroix as to the origin of the cordierite-norites, and have extended 
the group of contaminated gabbros to include many diverse kinds 
of assimilation rocks. 
A. N. Winchell * described the occurrence of cordierite-biotite- 
norites (““ muscovadytes ’’) along the contact of gabbro and schist 
in Minnesota; such contaminated gabbros are confined to the 
contact zone. Winchell considered the cordierite-norites to result 
from the absorption of aluminous material from the surrounding 
1 See H. H. Read, GEou. Maa., Vol. LXX, 1919, p. 370. 
2 W. R. Watt, loc. cit., p. 272. H. H. Read, Swmmary of Progress for 
1918, 1919, p. 28. 
3 A. Lacroix, loc. cit. 
* A. N. Winchell, loc. cit. 
