200 STRATIGRAPHICAL GEOLOGY 



have been met with in borings beneath the Cretaceous Beds which 

 underlie London and Essex. They are known to occur in the 

 same position in Belgium, and they emerge again to the surface 

 in the Ardennes, so there is every probability of their being con- 

 tinuous across the intermediate areas. 



1. Devon and Cornwall 



The Devonian rocks in England have been so crushed, plicated, 

 and broken by faults that it is no easy matter to make out the 

 original order of succession either in North or South Devon, and it 

 is almost impossible to estimate their original thickness. It will 

 be noticed that the section, Fig. 66, shows a general inclination of 

 the beds towards the north, newer and newer beds coming on in 

 that direction. This is the rule in South Devon and North 

 Cornwall, and yet the prevalent dips actually observed and 

 recorded on the maps of the Geological Survey are toward the 

 south, which would lead one to infer a complete inversion of the 

 beds. These dips, however, are those of the minor plications, for 

 not only is the whole system flexured into a succession of curves, 

 but each flexure is frilled into smaller plications. 



The following account is compiled from the memoirs of the 

 Geological Survey on the new map of Cornwall and South Devon, 

 and from papers by Mr. W. A. E. Ussher, supplemented by some 

 observations of my own ; for North Devon the publications of 

 Jukes, Etheridge, Hicks, and others have been consulted. 



The rocks of both the northern and southern areas are divisible 

 into Lower, Middle, and Upper Series, and in order to deal with 

 each of these series separately we must first give a tabular view of 

 the subdivisions which have been made in each area. In con- 

 structing this table a group of rocks occurring in North Devon 

 and known as the Morte slates has been omitted, for reasons 

 which will be mentioned, and because the correlation of the other 

 groups is complete without them. On the map, Fig. 64, these 

 Morte slates are marked as a separate tract of Lower Devonian. 



Cornwall and South Devon. i North Devon. 



