54 Scientific Proceedings, Royal Dublin Society. 



In its general eiiaracter, the specimen has very little of the 

 appearance of a crushed igneous rock; on the other hand, it 

 closely resemhles the normal mica schists produced by the altera- 

 tion of Ordovician slates in contact with granite, and since manga- 

 nese is known to occur in large quantities in association with the 

 Ordovician rocks of North Wales, I am strongly inclined to suspect 

 that this hornfels has been produced by the contact metamorphosis 

 of a manganese-bearing slate, which we have every reason to 

 suppose was of Ordovician age. 



