214 VOLCANIC ROCKS OF COEITWALL. 



the volcanic matter has Lroken through what I take to be 

 Devonian formation; at Porthollow the serpentines and 

 hornblendic gneiss rest on the lo^er Silurian ; at Nare Head, 

 Yeryan, the serpentines and hornblendic slates rest on the 

 lower Silurian ; at Duporth it pierces the lower silurian ; and 

 at Menheniot, I think, the rock it pierces is now considered 

 Silurian also ; therefore I cannot think that this volcanic activity 

 can be placed at an earlier date than the upper Silurian or 

 Devonian times. 



