57 



in this outcrop in search for the paleozoic rocks proved the clayey 

 oolites to attain considerable thickness, some 1,000 feet of Kim- 

 meridge Clay being followed by an almost equal thickness of 

 Oxford Clay, in which the boring was finally abandoned. The 

 later borings for coal in the Kent area have added to our know" 

 ledge of the extent and thickness of the concealed Kiinmei.dge 

 Clay. At Penshurst it is over 600 feet thick. Nearer Dover the 

 thickness rapidly diminishes, in some places being altogether 

 missing, as if the p re-cretaceous denudation had been there very 

 active. It is interesting that what appears to be the greatest 

 thickness of Kimmeridge Clay should be within a short distance 

 of the shore of the Kimmeridgian sea. Evidently, the sinking of 

 this sea floor was much more rapid and continuous there than 

 further out to sea. Another striking feature is that it is practi- 

 cally coincident in extent with the region of the greatest upheaval 

 which, in Miocene times, developed the Wealden anticline 



The Essex-Kent basin probably covers, the greater part of 

 the counties of Kent, Essex and Cambridge, and is divided 

 geographically by the Thames estuary into two areas. 



Jurassic rocks generally are well developed 'n France, forrmng 

 at tne surface a rough figure 8, the northern circ/e practically en- 

 closing the Paris Basin and the southern ringing the paleozoic and 

 igneous rocks of the Central Plateau. Kimmeridgian rocks con- 

 tribute much to the topographical features of the eastern borders 

 of the Paris Basin against the Ardennes and the Lorraine plateau. 

 The harder oeds form the western heights of the Meuse near 

 Verdun. Kimmeridgian rocks also form a most interestmg part 

 of the extension of the English Weald into the the district 

 immediately round Boulogne, there forming a grand arc of cliffs 

 enclosing the area a few miles outside the town. Here the for- 

 mation consists of sandy shales, clays and limestones, and is 

 covered by Portlandian rocks. A similar fades of the formation 

 is developed near the Meuse. 



Leaving the question of the geographical distribution of the 

 Kimmeridge Clay we now come to consider its stratigraphy. The 

 thickness of the clay varies considerably from district to district 

 in its range across England. It is thickest in the South Coast 

 exposures and borings, but, like the other stages of the Upper 

 Jurassics, it becomes notably thinner in the Midlands, again in- 

 creasing in Yorkshire. This variation in thickness is in all proba- 

 bility due to the irregularity of the rate of deposition of the 

 constituent material upon the floor of the Kimmeridgian sea. This 

 irregularity in turn mav be connected with either a change in the 

 rate of subsidence of the floor from district to district, or a local 

 variation in the supplv of silt carried dowi from the bordering 

 lands by rivers according to the shifting of their estuaries. The 

 prevailing sombre colour of the deposits is sometimes ascribed to 

 rocks, exposed and wasted in Kimmeridgian times, beinp- in all 

 probability coal measures and the slates of the still older Silurian 

 and Ordovician rocks of the Wales of that time, and of the 



