part 4] pexholog r or the arnage district. 485 



Discussion. 



Dr. J. S. Flett said that the Author's work and the previous 

 work of Mr. W. R. Watt had shown that in Aberdeenshire and 

 Banffshire we had probably the best example of a province of 

 contaminated basic igneous rocks anywhere at present known to 

 exist. The speaker had had opportunities of studying these rocks, 

 both in the field and in the laboratory. With the Author's con- 

 clusions regarding contamination he was thoroughly in sympathy ; 

 but, when the Author attempted to show that as end-facies of a 

 contaminated series nearly normal rocks might arise, the speaker 

 confessed that he felt unconvinced, and inclined to the opinion that 

 the xenoliths described could hardly be regarded as a satisfactory 

 mechanism for the chemical exchanges required. 



Dr. C. E. Tillet regarded the production of norites and 

 cordierite-norites by the contamination of gabbroid magma as long 

 since established by the observations of Lacroix, Winchell, and 

 Watt. The mechanism of the process was essentially a reaction of 

 the monoclinic pyroxene with aluminium silicate to produce 

 hypersthene and anorthite, the former mineral reacting again with 

 excess aluminous material to produce cordierite. The recognition 

 of norite and cordierite -norite in the Arnage mass as contaminated 

 rocks was, therefore, well founded. By some writers the anorthosite- 

 charnockite family of rocks was denominated a magnesian group ; 

 but the analyses of the various members of the family did not 

 support this contention. The presence of rhombic pyroxene in 

 them was favoured by a high alumina content, despite low 

 magnesia and often high lime. The norite of the Arnage mass, 

 however, was a rock strikingly rich in magnesia, and the speaker 

 concurred with the Author in his interpretation of it as a highly 

 contaminated product. The Author, however, went farther, and 

 derived a normal granitic type by contamination. The solution 

 of the problem of the origin of this granite must finally turn on 

 the interpretation of the field evidence ; but the speaker Avas 

 impressed by the freedom of the granite as a whole from xenoliths, 

 a fact which appeared to need careful consideration when the origin 

 of the granite was examined, and he desired to hear the Author's 

 opinion with reference to an alternative view of the granite as a 

 separate intrusion into the roof. 



He enquired whether corundum had been discovered as a 

 contamination-product of the Arnage mass. Corundum was an 

 abundant mineral constituting xenoliths of the Cortlandt Series 

 as described by Williams & Rogers ; but it was a remarkable fact 

 that cordierite-norites were not recorded from that area, a point 

 demanding further investigation. Was corundum found in direct 

 contact with olivine or hypersthene ? The recent work of S. G. 

 Gordon & A. L. Hall on corundum-bearing rocks from North 

 America and South Africa tended, in the speaker's opinion, to 

 throw doubt on the manner of origin of the now classic 



