172 DREDGINGS OF THE MAWNE BIOLOGICAL ASSOCIATION: 



associated with Dartmoor rocks, in the cletrital deposits lying on 

 the limestone at Cattedown, and examined by E. N. Worth; have been 

 found by the writer, again associated with Dartmoor rocks, on the floor 

 of clay-filled fissures in the Plymouth Limestone 20 feet below low 

 water, and have been found by him on the rock beds of the Plymouth 

 estuaries, buried beneath the silt. 



As a result of the dredgings a considerable westerly extension of 

 the boundary lines of the Trias, the Lias, and the Cretaceous must be 

 made on our maps, beyond the present usually accepted speculative 

 bounds. And the theory of an Eocene drift, sometimes put forward to 

 account for the flints, must be abandoned. 



It appears that from distant geologic time a depression has existed, 

 having the same trend as the western part of the English Channel, 

 and occupying a part at least of the same area. The New Eed Sand- 

 stone first distinctly shows the previous existence of this depression. 

 From Torbay to Plymouth the northern verge of the New Pted de- 

 posits touches the present shore-line here and there ; always the 

 derived fragments in the conglomerates and sandstones are largely 

 from local rocks. From Plymouth to nine miles south-east of the 

 Lizard it runs parallel to the coast without absolutely touching it, 

 and how far further west it extends we do not at present know. An 

 arm of the great inland sea of this period, probably of its later or 

 Triassic years, had its northern shore much where the waters of the 

 Channel now meet the cliffs of Devon and Cornwall. How wide the 

 Trias lake was along this western extension cannot at present be 

 known ; its deposits are lost under those of the succeeding Liassic sea, 

 perhaps to reappear nearer France, perhaps not. 



During the later Jurassic period this depression would appear to 

 have slowly risen free from the waters, and in part, if not in whole, to 

 have become a subaerial valley. 



The Cretaceous era witnessed its entire submergence, although the 

 highest points of Devon, where Dartmoor and Exmoor now stand, may 

 have appeared as islands above the surrounding waters. 



This submergence was gradual. A problematic coast-line of the time 

 of the Lower Chalk has been laid down by Mr. Jukes-Browne.^ By 

 that author's consent the map accompanying his paper in the Transac- 

 tions of the Devonshire Association is here reproduced (Text, fig. 4). 



It may be that the westerly extension of the Cenomanian sea has 

 not been sufficiently prolonged ; be that as it may, the sea of the Upper 

 Chalk sent an arm westward to the Lizard parallel or probably beyond. 



1 "Devonshire in the Time of the Lower Chalk," Trans. Dev. Assoc, Vol. XXXV, 

 1903, p. 787 et seq. 



