62 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



some time subsequent to the accumulation of the uppermost coal- 

 measures ; allowing time too for the accomplishment of all those 

 changes in the beds of vegetable matter, as well as in the associated 

 strata, whereby they became converted into mineral coal and crystal- 

 line limestones. 



This line of disturbance traversed the district of the coal-growths, 

 from E. to W., from the Boulonnais, far into the mid-European 

 area. 



To those who adopt implicitly the notions of M. Elie de Beau- 

 mont as to the correspondence between the ages and directions of 

 linear disturbances, and as to the dependence of the obvious physical 

 features of the earth's surface on such disturbances, it would be suf- 

 ficient that we should here point to the structure of the S.E. portion 

 of England* to satisfy them that it beloDgs, in all its incidents, to 

 that band of flexures and disturbance which ranges from the Bou- 

 lonnais eastwards ; and the presumption based on such external phy- 

 sical characters would acquire the value of a certainty, when, as at 

 Frome, the great sheet of Carboniferous Limestone, of the age of that 

 exposed in the Boulonnais, is seen to emerge from beneath the Chalk 

 axis with exactly the same relations to the Oolitic group, and having 

 the Coal-measures in convenient proximity to the surface on the N. 



The study of the structure of the axis of Artois suggests thus 

 much, that there is a strict coincidence between the disturbances 

 which its oldest and newest sedimentary groups present : this is a 

 subject which might be extended to any length, both from its 

 importance in physical geology, and from the abundance of illustra- 

 tion which this particular line of country affords ; enough, however, 

 for the results of the present inquiry will suggest itself to any one 

 who can bring to this study of a good physical map a competent 

 knowledge of the geological structure of the country from the Rhine 

 to the West of England between the 50° and 52° N. lat. 



The general law seems to be, that when any band of the earthy 

 crust has been greatly folded or fractured, each subsequent disturb- 

 ance follows the very same lines, — and that, simply because they are 

 the lines of least resistance. 



In this way, marked physical features in any region become un- 

 erring guides as to the character and extent of the earliest disturb- 

 ances which took place there : the sharp axis of Artois is continued 

 across our area by the range of the North Downs and those of Hants. 

 There is this feature to be observed along the whole of this line, that 

 on the north limit of this ridge the beds dip suddenly and rapidly, 

 and hence a line of fractures, extending from near Arras to the E. 

 end of the Boulonnais, and from the N.W. point of the Wealden 

 denudation to the valley of Devizes. This relative depression on the 

 north has preserved the Nummulitic series (Lower Tertiary), just as 

 along the Franco-Belgian line the depression on the north was the 

 cause of the preservation of the great coal-trough. Applying this 

 consideration to the structure of our area from Kent into Somerset, 



* See particularly Mr. Hopkins's map in vol. vii, of Trans. Geol. Soc, 



