142 Reports and Proceedings. 



of the dip of one of the higher beds, due to the stwi of the thinnings of the under- 

 lying 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 oar inferences as ta 

 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 Palseozoic rocks under London are at about 800 feet below sea-level. We 

 now know, by the sub-Wealden boring, that under the centre of the Weald the 

 Paljeozoics are also below sea-level. Supposing that they shoitld be found at 

 about the same depth as at Kentish Town, then the Palseozoic floor will 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 

 the area of water within which they were deposited had some limit to the north, 

 and that the Palseozoic 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 under- 

 gone. 



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. 



The beds which support Geological Basins frequently thin towards the centres 

 of those basins, thus producing, wholly or in part, the basined form of the strata. 

 It was, however, shown that the beds of the basins themselves frequently thicken 

 towards the centre of the basins. 



Discussion. — The Rev. O. Fisher remarked that he had always considered 

 that the Palaeozoic rocks under London had formed an axis against which the 

 Secondary rocks had abutted, instead of their being carried over the old rocks, as 

 shown in the diagram. He presumed that there would be difficulty in any natural 

 cause constantly leading to the thickening of strata at some particular spot so as 

 to form a ridge in a certain position ; and argued that the shattered condition of 

 the flints in some tilted rocks showed that they had been violently upturned. 



Mr. Seeley thought the paper extremely suggestive, though possibly its sugges- 

 tiveness had been carried too far. If the author's views were correct, the sea in 

 which the beds had been deposited must have been of enormous depth, but of this 

 we had no evidence. He could not believe that the chalk or any other sedimen- 

 tary deposit could in the process of deposition assume such dome-like forms as 

 would be necessary under the author's hypothesis. 



Prof Hughes considered that there were two kinds of thickening, ( i ) by deposits 

 from a sliore line, or (2) by sediment accumulating in a basin. The instances 

 adduced appeared to be the result of the thickening of strata in a basin. He 



