Vol. 65.] OVERTHEUSTS AI TINTAGEL. 279 



helped me with many valuable suggestions, and encouraged me to 

 continue the work that I had commenced. Mr. George Barrow 

 has also kindly assisted me ; and I wish to thank Mr. Maynard 

 Hutohings and Mr. John Parkinson for their ready loan of micro- 

 scope-slides of the' rocks of the district. 



EXPLANATION OF PLATE XIII. 



Geological map of a portion of North Cornwall, on the approximate scale 

 of 8 inches to the mile. 



Discussion. 



Mr. Clement Reid congratulated the Author on having worked 

 out the structure of a very difficult area. Overth rusting in Cornwall 

 seemed to have taken place during at least two periods — before and 

 after the intrusion of the granite-masses. Could the Author say 

 what was the date of the Tintagel overthrusts? The Author had 

 shown that the cleavage had been developed prior to the granite- 

 intrusion, for the metamorphic minerals lay on cleavage-planes. 

 Was there any trace of metamorphism acting also on the breccia of 

 the thrust-plane ? 



Mr. W. A. E. Usshek congratulated the Author on the successful 

 working-out of an Upper Devonian district in which, contrary to 

 his own experience in Devon and other parts of Cornwall, litho- 

 logical types were sufficiently marked and persistent to show so 

 clear a sequence of minor subdivisions that thrusts and disturbances 

 could be traced out and their effects clearly indicated. 



The Upper Devonian age of the rocks in this part of Cornwall 

 was first claimed by him (the speaker) in 1891, in a tentative paper 

 based on the new reading of the Devonian succession that he had 

 successfully applied to the Torquay and Newton Abbot area. 



The effects of the thrusts described by the Author, in bringing 

 different members of a sequence into abnormal juxtaposition, 

 emphasized the caution necessary in working out the relations of 

 the Culm and Upper Devonian in the area between the district 

 described in the paper and Tavistock, where the speaker had 

 encountered during traverses appearances at variance with the 

 general sequence manifested elsewhere. 



He enquired whether the Author had found fossils in the beds 

 above the Delabole Slates with Spirifer verneuili, and cited Upper 

 Devonian Slates near Beer Ferrers and Holne Bridge near Ashburton, 

 where Spirifer verneuili was met with, to the apparent exclusion of 

 all other forms. The relative positions of these and other types of 

 the Upper Devonian to the Lower Culm rocks had led the speaker 

 to regard an unconformity between these groups as highly probable ; 

 and if the beds above the Delabole Slates were below the 

 Petherwin Beds, the apparently unique occurrence of the latter 

 might be due to concealment by unconformable overlap elsewhere. 



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