1859.] HULL — THINNING- OUT OF THE SECONDARY ROCKS. 81 



EXPLANATION OF PLATE IV. 



The sections in Plate IV. are intended to illustrate the thinning-out towards 

 the South-east (and, per contra, the thickening towards the North-west) of all 

 the formations from the base of the Great Oolite down to that of the Trias. 

 Though founded on actual admeasurement of the strata, the sections are not 

 drawn to any true horizontal scale. 



Fig. 1 shows the thinning-away of the Liassic and Oolitic strata, along a lino 

 drawn from the banks of the Severn, near its junction with the Avon, across 

 Bredon Hill, the Vale of Cheltenham, the Cotteswold promontory, and Vale of 

 Moreton, to the hills of Coral-rag above Oxford. The line runs W.N.W. and 

 E.S.E. By comparing the thickness of the formations (as given in the figures 

 immediately under the sections) where they are capable of being measured, it 

 will be seen how they all tend to thin away towards Oxford. The whole of the 

 Inferior Oolite is not present on Bredon Cloud, so that its thickness cannot bo 

 compared with that of its representative on the Cotteswold Hills ; but in the Valley 

 of the Cherwell, near Woodstock, the formatiou has altogether disappeared. 

 The Upper Zone of the Great Oolite, however, evinces no tendency to thin away 

 in any direction. 



Figs. 2, 3, and 4 may be considered as one interrupted section, and are to 

 illustrate the same principles in the case of the Triassic rocks. The direction 

 of the section is along a line drawn from the mouth of the Mersey to that of 

 the Thames ; and in each the thickness of the formation with its groups is shown 

 as it occurs in Cheshire, Staffordshire, and East Warwickshire. It will bo 

 observed that, wliile the Bunter Sandstone attains in Cheshire and Lancashire 

 the surprising thickness of 2000 feet and upwards, in Staffbrdslure it has con- 

 siderably lessened, and dies out before reaching the neighbourhood of Coventry. 



The Keuper series in Cheshire is upwards of 3000 feet thick ; in Staffordshire 

 it is only 1200 ; and in East Warwick about GOO. The section is continued 

 through the Liassic and Oolitic formations into Oxfordshire. 



These sections also show the manner of the distribution of the Lower Permian 

 rocks along the same line of country. This formation becomes more fully de- 

 veloped in Warwickshire than in Staffordshire, and in this latter comity more 

 than in Cheshire ; proving the remarkable change which ensued at the close 

 of the Palaeozoic period in the physical circumstances under which the rocks 

 of the two great epochs were deposited. 



VOL. XVI. — PART I. 



