W. A. E. Ussher — Devonian Rochs of Cormmll. 589 



On the other hand, in South-East Cornwall it became evident that 

 the Looe beds had been traced into Upper Devonian on their strike, 

 through the great structural fault hetioeen them having escaped detection. 



The results of this critical examination were contributed to the 

 Eoyal Geological Society of Cornwall and appeared in theTransactions 

 of that Society in 1891. The paper was entitled "The Devonian 

 rocks as described by De la Beche interpreted in accordance with 

 recent researches." It was accompanied by a map reduced from 

 the 1 inch on which, after tracing De la Beche's minor sub- 

 divisions with infinite pains, as far as the information obtainable 

 from the Keport enabled me to do, I had remodelled them and referred 

 them to widely different horizons. This map shows the relations of 

 the Upper, Middle, and Lower groups of the Devonian for the first 

 time, and with approximate general accuracy ; it also furnishes the 

 key to the Devonian geology of Cornwall. 



Having since mapped the country from Plymouth to St. Austell 

 in detail, I have found the inference of great structural faulting to 

 be correct, the effect being produced by two or three faults, one of 

 which runs in a north-westerly direction from Cawsand, as shown 

 in the little map illustrating the results I arrived at in 1891. By 

 the discovery of the Upper Devonian Entomides, StyliolcB, and traces 

 of the Biidesheim fauna, I have proved the occurrence of Upper 

 Devonian rocks where shown on the little map east of Liskeard. 



In North Cornwall, the discovery of the Biidesheim fauna by 

 Mr. Fox at Trevone and the researches of Mr. Parkinson have proved 

 the general correctness of the map as showing the extension of 

 Upper Devonian. The general line of the Lower Devonian outcrop 

 is correct as far as the occurrence of the Staddon grits could be 

 inferred from the materials in Chapter iii. 



These results were achieved by analogy with a carefully mapped 

 district, and they are directly contrary to De la Beche's correlations. 

 As regards the run of the Lower Devonian subdivisions, the map 

 had to be constructed from totally inadequate materials, so that the 

 relative outcrops of the Dartmouth slates and Looe beds, in the 

 absence of analogy, were very incorrectly shown. 



An alternative rendering of the structure of Watergate Bay was 

 given, showing the Dartmouth slates as an anticlinal, although not 

 suggested by De la Beche. This structure has since been worked 

 out by Mr. Eeid. 



I took the Grrampound rocks as a base to the Devonian on account 

 of their conglomeratic character and because there is nothing in the 

 Report to show that they partake of the south-westerly strike of the 

 rocks to the south, so that they appeared to mark the unconformable 

 junction of a newer with an older group. 



The revolutionary character of my paper of 1891 was pointed out 

 by the late E. N. Worth in an article which appeared in the same 

 Transactions (Eoyal Geol. Soc. Corn.) for the ensuing year, 1892. 

 In it he criticized adversely the structure put forward, but in a most 

 fair and candid manner. He corrected an error as to the Modbury 

 «lvan, which in subsequent mapping was found to be a granophyre. 



