Vol. 60.] AX EOCENE OUTLIER OFF THE CORNISH COAST. 11 



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sides. The notch is the plaue of marine denudation which was 

 formed in Pliocene times near the sea-level ; the parts of the 

 pre-existing valley below that level, either remained unaltered, or 

 tended to rill up with Pliocene sediments, outlying patches of 

 which are still preserved. 



The occurrence of an Eocene outlier at a low level opposite the 

 southern end of the valley just described, suggests that the river 

 Mowed from north to south (more exactly from north-east to south- 

 west) ; but this evidence is by no means conclusive, for we must 

 remember that Miocene earth-movements played an important part 

 in the Hampshire Basin, and disturbances of this character may 

 have extended into Cornwall, quite altering the drainage. 



The little evidence vet available suggests that Eocene rivers 

 radiated from the high ground of Dartmoor, flowing westward as 

 well as eastward, and that one of these rivers turned southward to 

 cut through the depression leading to Mount's Bay. 



Whether the Eocene gravel which seems to occur beneath the 

 waters of Mount's Bay is a mere isolated patch or no is not clear. 

 It may be an outlier belonging to an extensive Tertiary basin 

 underlying the western part of the English Channel, and comparable 

 in importance to the Tertiary basin of Hampshire. I would not 

 venture to make this suggestion, were it not for a piece of evidence 

 which cannot be ignored in a paper dealing with the Eocene geology 

 of West Cornwall. Some 20 miles south-west of St. Michael's 

 Mount lies the isolated mass of phonolite known as the Wolf Bock, 

 on which stands the* celebrated lighthouse. This volcanic rock has 

 been a standing enigma to the geologist : there is nothing like it in 

 Cornwall, the only lavas of this type in Britain being of Palaeozoic 

 date, and occurring in Ireland and Scotland. On the Continent the 

 phonolites are mainly Tertiary. It has already been suggested by 

 more than one writer that the phonolite of the Wolf Kock may be 

 of Eocene date, for, unlike the Palaeozoic igneous rocks of Cornwall, 

 it is neither altered nor sheared. 1 The occurrence of a lava of this 

 exceptional type, thus placed in the course of an Eocene valley, 

 may perhaps be pointed to as a confirmation of the view that an 

 Eocene basin may lie under the sea in the western part of the 

 English Channel. 



Discussion. 



Prof. W. W. Watts observed that one of the many interesting 

 points of the paper just read was connected with the occurrence of 

 phonolite at the Wolf Bock, at one time regarded as unique in the 

 British area. Geologists had long been searching for evidence as to 

 the date to which it could be assigned. However, Dr. Hatch had 

 since discovered Carboniferous phonolite in the South of Scotland, 

 and the speaker had discovered one associated with Carboniferous 



1 S. Allport. " On the Microscopical Structure k Composition of a Phonolite 

 from the Wolf Rock' Geol. Mag. 1871, p. 247 : and ' Note on the Phonolite from 

 the Wolf Rock ' ibid. 1874, p. 462. 



