7-1 Geological Society: — 



have an appreciable effect in producing or increasing its own apparent 

 dip ; but where a whole series of beds thin constantly in one direction, 

 the amount of the dip of one of the higher beds, due to the sum of 

 the thinnings of the underlying beds, is often very considerable. 



In illustration of this the author indicated the Lower Jurassic 

 rocks between Leckhampton Hill and Burford. The Inferior Oolite, 

 and the Upper, Middle, and Lower Lias thin out rapidly to the east 

 along this line ; the base of the Middle, Lias is nearly a horizontal 

 line, the easterly " dip " of the Great Oolite being due to the easterly 

 thinning of the Middle and Upper Lias and the Inferior Oolite. The 

 base of the Lower Lias has a westerly dip. 



It is generally supposed that the dip of any bed is due to great 

 movements of the earth's crust ; from the facts mentioned the author 

 argued that our inferences as to such movements will vary according 

 to the beds which happen to be exposed at the surface. In the 

 example given we assume a westerly upheaval because we see the 

 Great Oolite dipping to the east. If over this area the whole of the 

 higher strata had been removed down to the Middle Lias, we should 

 perceive the beds to be flat ; if the denudation had exposed the base 

 of the Lias, we should probably suppose that along this line there 

 had been an easterly upheaval. 



Under London the entire series of strata between the Palaeozoic 

 rocks and the base of the Upper Cretaceous is absent ; as we recede 

 from the London Basin the intermediate beds necessarily come in 

 and gradually thicken, producing the dip of the Cretaceous beds 

 towards the London Basin. 



The Palaeozoic rocks under London are about 800 feet below 

 sea-level. We now know, by the sub-Wealden boring, that under 

 the centre of the Weald the Palaeozoics are also below sea-level. 

 Supposing that they should be found at about the same depth as at 

 Kentish Town, then the Palaeozoic floor wdll be approximately a 

 straight line, whilst we know that the whole of the Wealden and 

 other cretaceous beds dip to the north, their dip being thus wholly 

 accounted for by the gradual thinning out of all the beds as they 

 approach London. In the case of the Weald, some of the strata 

 are of freshwater origin; we must suppose that tbe area of 

 water within which they were deposited had some limit to the 

 north, and that the Palaeozoic floor, which is now approximately 

 flat, rose up to the north as a bounding ridge. But even if this be 

 so, it is evident that the present dips are no exact measure of the 

 amount of upheaval which the beds have undergone. 



Other examples were quoted in which the observed general 

 dip corresponds in direction with the known or inferred general 

 thinning ; and it was shown that in all such cases we are liable to 

 serious error in inferring the amount of upheaval from observed dips. 



It is, however, evident, from the faults intersecting strata, that 

 upheavals and disturbances have taken place ; but unless we assume 

 every bed to have been deposited on a perfectly horizontal plane, we 

 cannot infer the amount of such upheaval from the present position 

 of the bed. In all cases we must take into account the actual or 

 possible thinning of underlying beds. 



