part 4] 



petrology of the arnage district. 



447 



Dr. A. Harker was considered to have a more specialized meaning, 

 since it has come to denote rocks formed either ' by the mixture 

 of two magmas, or by the assimilation of a rock already con- 

 solidated by the magma of a later intrusion.' : Perhaps, too, 

 I was influenced by the emphasis laid by Dr. Harker on the 

 barrenness of these hybrids in their wider petrogenetic aspect, and 

 so endeavoured to remove contaminated rocks from these sterile 

 relatives. 



Fig. 1. 



In the Hun try and Insch masses the contaminated rocks, 

 although occurring at many localities, covered in no place much 

 more ground than a square mile ; besides, they did not appear to 

 throw any certain light upon the petrogenetic problem. But in 

 the third of the masses to be investigated — that of Arnage and 

 the subject of this communication — it is gratifying to find that 

 contaminated rocks form the main visible part of the intrusion, 

 and are seen over an area of about 16 square miles. At Arnage 

 the scale of the phenomena is large enough, and the stage reached 

 in contamination is sufficiently far advanced, to warrant, I believe, 

 certain speculations as to the relation of contamination to the 

 origin of the diversity of igneous rocks. 



1 A. Holmes. 'The Nomenclature of Pctrolog-y ' 1920, p. 121. 



