588 W. A. E. Ussher — Devonian Rocks of Cornwall. 



fossiliferous Looe beds) were taken as the probable westerly con- 

 tinuation of the Plymouth limestones. The latter error was endorsed 

 by me in recording coast observations some years before I had any 

 practical acquaintance with the Devonian rocks of South Devon. 



As regards North Cornwall (see p. 91), we read "tlie slates of 

 Watergate Bay rest on the sandstones of St. Eval and St. Breocks 

 Down ; and the red and variegated beds of St. Kew and St. Minver 

 seem to be the same with the red and variegated beds of Watergate 

 Bay ; the calcareous rocks of Lower St. Columb Forth, Newquay, 

 and the Gannel representing in a general manner the limestones of 

 Eock, Padstow, and Permizen." 



The anticlinal of St. Breocks Down is elsewhere referred to in 

 'Chapter iii, and as these grits do not crop up to the north, we are 

 obliged to assume their identity with the Staddon grits. 



It will thus be apparent that, as regards South-East Cornwall, 

 De la Beche correlated the Looe beds^ of the Looe district with 

 rocks in the Saltash district which I have proved to be Upper 

 Devonian, and the same Looe beds in the Tregantle district with 

 Middle Devonian rocks in the Plymouth district. As regards North 

 Cornwall, it will be seen that De la Beche did not treat the 

 variegated slates of Watergate Bay as an anticline, but by assigning 

 that structure to the Staddon grits he correlated the Lower Devonian 

 rocks on the south with the Upper and Middle Devonian rocks on 

 the north. 



The Devonian geology of Cornwall prior to 1890 was thus left in 

 a state of chaos. In that year, after two years' careful mapping of 

 the Torquay, Totnes, and Newton Abbot area, I endeavoured, by 

 a thorough perusal of De la Beche's Chapter iii with the maps 

 before me, to follow his minor subdivisions, and to test his corre- 

 lations in order to ascertain how far they jnight be made to agree 

 with the subdivisions I had established by detailed survey. I then 

 found that there was no escape from the conclusion that the grits of 

 St. Eval and St. Breocks Down and of Boconnoc represented the 

 Staddon grits which at Mount Edgecumbe are on the mean latitude 

 of their South Devon outcrop. The difference in latitude I conceived 

 to be due to the existence of a great north-westerly fault from 

 Cawsand, cutting off the Upper and Middle Devonian roclcs on their 

 westerly strike against the Lower Devonian subdivisions and shifting 

 the outcrop of the Staddon grits northward. 



I had recognized the lithological affinities of the variegated slates 

 of Mutley to the red and green Entom-yielding Upper Devonian 

 slates of Ansteys Cove, Goodrington, and High week (Newton Abbot) 

 in 1888. So that it became evident that the red and variegated 

 slates to the north of the Staddon grits had been confounded with 

 the Dartmouth slates in North Cornwall, and that the anticlinal 

 structure assigned to the grits of St. Breocks Down was suggested 

 by this similarity. 



1 Referred to the Siegener Gramvacke by Dr. Kayser, who established the 

 Taunusian age of the fauna in 1882. 



