TRANSACTIONS OP THE CECTIONS. 67 



On the South'Sastefly Attenuation of the Oolitic, Liassic, Triassic, and 

 Permian Formations. By Edward Hull, A.B., F.G.S. 



The subject of this paper is partly of a local, and partly of a general character. 



Sir R. I. Murchison having called the attention of the Section to the Map of the 

 Geological Survey (No. 44) just completed, and embracing the region of the Cottes- 

 wold Hills, together with the liassic plains of Gloucester and Moreton, the author 

 proceeded to point out the remarkable diminution in thickness which the rocks of the 

 Cotteswold Hills undergo in their extension to the borders of Oxfordshire. 



Taking as points of comparison Leckhampton Hill near Cheltenham, and Burford 

 in Oxfordshire, distant from each other about twenty miles, it was shown that the 

 same beds, which at the former locality have an aggregate thickness of 624 feet, at 

 the latter have dwindled down to the twenty-ninth part of this amount. The for- 

 mations included in this coiaiputation range from the marlstone to the Fuller's earth, 

 and may be tabulated as follows : — 



Leckhampton Hill. Bvirford. 



Feet. Feet. 



Fuller's earth 25 absent. 



Tf . rRagstones 38 10 



Inferior „ 5_ „, /:„.,..,i: i,-^ n loo 



oolite 



■1 



Freestones (including oolite marl) . . 188 absent. 



Pea grit 38 absent. 



Ferruginous sands, &c 20 ? absent. 



Upper lias shale 200 5 



Marlstone, or middle lias 115 6 



Total 624 21 



From this table it would be observed that the ragstones of the inferior oolite, in- 

 cluding a bed remarkable for the abundance of Clypeus sinuatiis, forms the most con- 

 stant zone of the inferior oolite, and that at the eastern limits of the district it is the 

 sole representative of the formation. This fact tends to show that this terminating 

 zone was deposited in a sea of greater depth and tranquillity than that of the lower 

 members of the formation ; an hypothesis, which is also borne out by differences in the 

 state of the included organisms. For while i\\e freestones everywhere present the 

 phsenomena of false bedding, and are to a great extent composed of organic^ debris, 

 the stratification of the ragstones is always regular, and the organic remains in good 

 preservation, though frequently occurring as moulds and casts. 



Passing on to the consideration of the formations which underlie the marlstone or 

 middle lias, Mr. Hull proceeded to show from analogy the strong probability that the 

 lower lias forms no exception to the law of easterly attenuation, which obtains in the 

 case of the upper and middle members of the liassic group ; and that consequently 

 under Burford the lower lias would be found of comparatively small depth. From 

 these premises, he also drew the conclusion, that further in the same direction, e. g. 

 under the city of Oxford, all the strata already alluded to must be on the point of dis- 

 appearing. 



It was next shown that the trias of Central England undergoes a similar south- 

 easterly attenuation, so that, while in Lancashire, Cheshire, and Shropshire, the 

 Keuper and Bunter attain their greatest development, in the counties to the eastward 

 bordering on the lias, these formations are greatly reduced in thickness. With regard 

 to the Permian formation, it was not possible to speak with equal certainty, as it has 

 been found, through the researches of the Government Geological Surveyors, to vary 

 rapidly in thickness. Thus while it is almost or altogether absent around the Leices- 

 tershire coal-field, it appears in considerable force on the flanks of the coal-field of 

 Warwickshire. 



Attention was then called to the fact, that on the borders of France and Belgium, 

 and in the " Bas-Bouloimais," all the secondary formations between the coal-mea- 

 sures and the chalk in the former case, and the great oolite in the latter, are altogether 

 absent*. This fact was shown to bear out the hypothesis of the author, and to lead 

 to the supposition that under some parts of Oxfordshire and Northamptonshire the 

 coal-formation may lie at depths not inaccessible to human agency. 



* Description Geognostique du Baisin du Bas-Boulonnais, par M. Rozet, 1828. 



5* 



