IS6 THE PETROLOGY OF THE ARNAGE DISTRICT. [vol. lxxix, 



corundum-deposits associated with peridotites in North Carolina, as 

 enunciated hy Pratt. 



The Author, in reply, stated that the contaminated rocks of 

 the Arnage Type and their associated cordierite-xenoliths represent 

 only an early stage of the contamination-process, and it is reasonable 

 to consider that the xenoliths, when introduced into the underlying 

 initial gabbro-sheet, had a concentration of magnesia and lime com- 

 plementary to the concentration of ' granitic ' oxides in the Ardlethen 

 Type of product. It is thought likely that these xenoliths were, at 

 this stage, composed mainly of rhombic and monoclinic pyroxenes. 



In answer to Dr. Tilley, the Author stated that, not only did the 

 field-evidence show that the granitic end-product was part of the 

 contaminated rocks, but also bands and patches of similar granitic 

 material occurred throughout the Arnage Type of rock. In reply 

 to Dr. Tille} r 's emeries concerning spinel and corundum, it was stated 

 that the spinel of the granitic end-product occurred associated 

 with cordierite patches. Corundum had not yet been found at 

 Arnage, but had been noted at Huntly in a cordierite-norite of con- 

 taminated origin. The Author was gratified to receive Dr. Tilley's 

 support in the view that the magnesia-rich norite of Arnage was 

 by no means an average norite. 



