76 SECTIONAL ADDRESSES. 



of the base of the Lias ; moreover the areas of greatest interruption in 

 the succession do not in general coincide with the areas of maximum 

 elevation. 



If we examine a geological map of the south of England the effects of 

 the Miocene folding on the Cretaceous and later strata leap to the eye, 

 and the folds can be readily traced as far as the western limit of the outcrop 

 of those rocks. There appears, however, at first sight to be a remarkable 

 absence of folding in the Jurassic area west of the Cretaceous boundary, 

 as if the folding so evident in the south-east died away in a westerly direc- 

 tion. I have never been able to conceive of any reason why the folding 

 which has so strongly affected the Mesozoic and Cainozoic strata of the 

 south of England should suddenly cease at the western limit of the 

 Cretaceous tract. As a matter of fact, if the outcrops of the Jurassic 

 formations are carefully examined and due regard paid to the surface 

 relief, several shallow folds can be traced through the Upper and still 

 farther west the Middle Jurassic outcrops. We have seen, moreover, 

 that the Lias rocks have suffered considerable folding since they were 

 deposited, but there remains a doubt whether this had occurred before 

 or after the Cretaceous. 



There are two possible explanations of the apparent absence of folding 

 in the Jurassic outcrops. If a series of rocks which dip, say, to the east, 

 is later subjected to folding, along say east and west axes, the effect on 

 the outcrops will be less pronounced than if the beds were lying horizontally 

 before the folding took place. The Mesozoic rocks from the Dorset coast 

 to Wiltshire were obviously tilted eastward and subjected to erosion 

 before the deposition of the Cretaceous, since the base of that formation 

 oversteps westward and comes to lie in succession upon all the strata 

 from the Purbeck to the Keuper. This explains in part why the Jurassic 

 formations pursue a less undulating course than the base of the Cretaceous. 

 If it were not for the fact that the Mesozoic rocks had acquired an easterly 

 dip prior to the deposition of the Cretaceous, I believe that the effect of 

 the folds on the outcrops would be just as prominent as it is in the 

 younger rocks. 



Another explanation is suggested by the general characters of folded 

 regions. Just as there are anticlines and synclines in a direction transverse 

 to the axes of folding which are culminating points of the movement, so 

 along the axes there are culminating points where the pitch of the folds 

 changes over. In the general direction of the fold axes, these regions tend 

 to occur at fairly regular intervals, forming, as it were, a succession of 

 cols and domes on the folded surface. A close examination of any folded 

 area will show that in the neighbourhood of the dome culminations the 

 movement is concentrated on one or two axes of uplift, whereas in the 

 neighbourhood of the cols it is dispersed over many axes, no one of which 

 attains a marked predominance over the others. 



These characters are well illustrated by the folding of the Weald. 

 The anticlinal structures which have such a marked effect in the centre of 

 the Weald can hardly be recognised in the minor wrinkles that traverse the 

 extensive chalk plateau between Winchester and Eangsclere, where the 

 only fold of any magnitude is the sharp Kingsclere monocline. Farther 

 west, however, several well-marked folds reappear in the Cretaceous, 



