68 SECTIONAL ADDRESSES. 



the Market- Wei ghton axis, must not of course be ignored ; but these 

 appear to haA^-e been responsible for only minor irregularities in the 

 deposition of the strata. 



Most of these formations, with the exception of the Oxford Clay, are 

 variable in lithology and are probably of shallow-water origin. The 

 Oxford Clay, however, is a formation so uniform over its whole outcrop 

 that one would expect it to have occupied a wide area. As with the Lias, 

 it seems difficult to believe that it died away before reaching the Palseozoic 

 rocks in the west. Slow elevation of the western area may, however, have 

 been in progress pari passu with the subsidence of the floor of the 

 Jurassic sea, so that deposits laid down at one period may have been 

 removed at a later period. It is not impossible, in fact, that the clays of one 

 period may have furnished in part the materials for those of a later period. 



Since there are so many doubtful elements in the problem, it would 

 be unprofitable to discuss the possibility whether and to Avhat extent the 

 higher Jurassic formations encroached on the Palteozoic region to the west. 

 Hull, Ramsay, Lamplugh and others have directed attention to the marked 

 easterly attenuation of these formations, and particularly of the 

 Kimmeridge Clay, all of which behave in a manner similar to the earlier 

 Jurassic strata. 



Briefly, although it is probable that at least the great clay formations 

 of the Jurassic once extended well over the Palaeozoic areas, it does not 

 follow that they maintained their hold over that area until the close of the 

 Jurassic era. It is a fact that the Jurassic rocks, particularly in the south- 

 west of England and probably elsewhere, had acquired an easterly dip 

 before the deposition of the Cretaceous, but whether this happened in 

 one episode of movement between the Jurassic and the Cretaceous, or 

 whether it is the result of a progressive increase in dip of each Jurassic 

 formation due to a succession of uplifts in the west, cannot be determined 

 from existing data. A closer study of each of the Jurassic formations on 

 the lines of the investigation by Kitchin and Lamplugh on the Mesozoic 

 rocks of the Weald, might throw light on this problem and resolve many 

 of the existing uncertainties. 



It appears to be the general opinion of geologists who have considered 

 this matter that the Cretaceous sea spread widely over the western areas 

 of Britain. According to Jukes-Browne's palseogeographic reconstructions, 

 none of the Jurassic formations invaded more than the margin of the 

 Palaeozoic region, whereas the chalk sea is represented as having covered J 

 all but the higher summits ; and if the chalk sea invaded the region it ia\ 

 not unlikely that a considerable thickness of deposits would be formed in 

 that sea. Jukes-Browne was inclined to regard the residual elevations 

 that rise above the Welsh plateau as defining the margin of the Chalk sea 

 (Building of the British Isles, p. 335). Although he recognised (p. 334) 

 that ' the pre-Cretaceous contours of the country were very different from 

 those which it now exhibits, all heights have been greatly reduced by the 

 work of sub-aerial agencies during Tertiary times,' he appears to have had 

 no suspicion that the relative elevations above sea level of various parts 

 of that tract may have been radically different then from what obtains at 

 present. 



Strahan was led to postulate the former existence of a blanket of 



