1S59.] HULL THLNNLNG-OUT 01' THE SECONDARY ROCKS. 71 



If we trace these beds towards the borders of Oxfordshire, along 

 the Valley of the Evenlode, that is, in a direction E.S.E. from 

 Bredon Hill, we find a marked diminution in the thickness. At 

 Burford, Shipton, and Charlbuiy, the farthest points where the 

 thickness of the Marlstone can be ascertained, we find it to have 

 dwindled down to an average of 20 feet, or less than one-tenth of 

 its amount at Bredon. If, therefore, this attenuation continues in 

 nearly the same ratio, the formation cannot extend far beyond the 

 city of Oxford. It is remarkable, however, that the " Upper Bock- 

 bed " shows far less disposition to thin out than the underlying sandy 

 strata, which, in Gloucestershire form nine-tenths of the mass of the 

 formation. 



C. Upper Lias. — In the case of the Upper Lias Shale, the south- 

 easterly attenuation is even more decided. Its thickness at Bredon 

 Hill is nearly 400 feet* ; but at Leckhampton Hill it reaches 230 

 feet, including a few beds of superimposed sands. From this point 

 eastward the formation thins away; and at Stonesfield, in the 

 Valley of the Evenlode, it may be said to have finally disappeared, 

 being represented only by a band of shale four feet thick ; the In- 

 ferior Oolite resting almost immediately on the Marlstone " Bock- 

 bed," which has here become a valuable iron-ore t. 



As we can thus, from actual observation, prove the south-easterly 

 thinning-out of the Middle and Upper Lias, I think, bearing in 

 mind their close relationship to the Lower Lias, that we are 

 justified in inferring a somewhat similar thinning-away towards 

 the Thames Valley on the part of this earliest formed member, 

 though it has probably a more extended range. Should this sup- 

 position prove correct, the whole Liassic Series should be on the 

 point of disappearing below Oxford. 



I here wish to repeat that in the case of the Lias the line of 

 maximum attenuation follows an E.S.E. direction. A deviation of a 

 few degrees northward produces a considerable change in the 

 thickness of the beds. Thus in the neighbourhood of Chipping 

 Norton and Deddington, which are nearly due east of Cheltenham, 

 the combined thickness of the Upper Lias and Marlstone is not 

 less than 200 feet. 



4. Inferior Oolite. — Proceeding next to the consideration of the 

 Inferior Oolite, I take as sections for comparison those at deeve 

 Cloud and Leckhampton Hill at the west, and those of the Valley 

 of the Evenlode at the east. The formation at deeve Cloud is 

 probably not less than :'><"> feel thick, and that at LeokhamptOD 

 Hill 2I>4 feet. This latter, ever since the original survey of Mnr- 

 chison£, has been regarded as the typical section of the Inferior 

 Oolite, not only for Kngland. hut for Europe. It there consists of 



* Ascertained from a measured borisonmJ Motion about t" !><■ published by 

 the Qeologioa] Survey. Bredon Hill ifl the must north-westerly district when 

 the Upper Lias exists ; and its great thickness there ihowa the rapidity of tip- 

 thinninp-out towards Oxfordshire. 



t Bee Map of the Qeologioa] Bui 3 w 



; Prooeecunga Geo! Boo. rol i p 188 



