144 SIR. S. S. BUCKMAN ON THE BAJOCIAN [Feb. I9OI, 



ward gives a section drawa by W. Topley, from Leckhampton 

 Hill to Burford/ where this remarkable thinning-out is shown, 

 the Keuper strata being actually brought up at Burford to about 

 250 feet above sea-level, as in Hull's diagrams ! 



Prof. Hull seems to have assumed this general thinning ' from 

 analogy ' {op. cit. p. 15).^ He explains this rather more fully in other 

 places. He found, eastward, a disappearance of all middle and 

 lower beds of the Inferior Oolite, and a very great reduction in 

 the thickness of the Upper Lias. He then seems to have 

 'assumed' that all other Jurassic beds in the district thinned- 

 out proportionately — that because the Inferior Oolite was reduced 

 from 280 to about 20 feet on the east, therefore the Lower Lias 

 must be reduced from 600 to about 50 feet. The great mistake arose 

 from thinking that the difference in the thickness of the Inferior- 

 Oolite rocks had been produced by diminution in amount deposited. 

 It was really brought about by contemporaneous 

 erosions — the principal of which was the one that 

 occurred before the deposition of the Upper Trigonia- 

 grit. This erosion planed away the strata of the Middle and 

 Lower Inferior OolitL and of the Upper Lias in an easterly direc- 

 tion until the lowest limit is reached in the Vale of Moreton ; 

 and eastward of that the strata begin to reappear in succession — 

 for instance, the Upper Lias thickens, the strata of the scissi 

 hemera come in, and there are certainly other rocks of the Inferior 

 Oolite. 



The idea that the rocks of the Inferior Oolite or Upper Lias 

 thin gradually in passing from west to east must be kept quite 

 distinct from the idea that alterations in their thickness have been 

 brought about by what maybe called penecontemporaneous 

 denudation. In the former case, the supposition of analogous 

 and continuous thinning of Middle and Lower Lias might be 

 justified. In the latter case, it would be an unwarranted surmise. 

 And the former case does not fit the facts, as detailed in this 

 paper; for the greatest thickness of strata of the Inferior Oolite 

 yet met with is evidently in the neighbourhood of the Buggilde 

 Street, near Cutsdean Hill, not westward, at Cleeve Hill. 



In the neighbourhood of the Buggilde Street there is a develop- 

 ment of about 50 feet, or perhaps more, of the Snowshill Clay and 

 Tilestone series. Against that must be set a loss of about 30 feet 

 by removal of the Phillipsiana-WitchelUa series. So there is a 

 gain of about 20 feet, in that way. The actual thickness of the 



1 ' Geology of England & Wales ' 2ad ed. (1887) p. 280. 



^ [See also the same author's paper ' On the South-easterly Attenuation of 

 the Lower Secondary Formations of England ' etc. Quart. Journ. Geol. Soc. 

 vol. xvi (1860) p. 70, which I had overlooked when writing. There he says : 

 ' If it can be shown that there is a tendency on the part of the Marlstone and 

 Upper Lias to thin away towards the south-east, and that this attenuation takes 

 place within the range of actual observation, there will be sfrong grounds for 

 inferring a similar propensity on the part of the Lower Lias ; at least, it is 

 upon these grounds that I base the analogy.' — January 'I^nd, 1901.] 



