C— GEOLOGY. 67 



From the Dorset coast to the borders of Gloucestershire and Oxfordshire 

 these formations attain a thickness of nearly 1,000 feet ; they fall away 

 to about half this thickness in Buckinghamshire and Rutlandshire, while 

 they are absent in Suffolk. From Lincolnshire to North Yorkshire, 

 however, they increase markedly in thickness. Their behaviour is, there- 

 fore, similar in general to that of the Lias, except that there does not appear 

 to be any obvious reduction in these rocks on the line of prolongation of 

 the Mendip region. 



Both at the northern and the southern extremities of the Jurassic 

 outcrop all the formations hitherto considered probably conform to the 

 principle enunciated by Lamplugh, that the present outcrop coincides 

 approximately with the areas where these formations attained their 

 maximum thickness. It is doubtful, however, whether this principle 

 holds good for the intervening region. 



The curvature in the strike of the Jurassic outcrop between Gloucester- 

 shire and Yorkshire is repeated in the base of the Cretaceous, but its 

 amount in that formation is even greater. There is no doubt that the 

 form of the outcrop is due to post-Cretaceous folding in consequence of 

 which the Mesozoic rocks have been arched up along a broad area, extend- 

 ing from the Midlands to East Anglia. The less pronounced effect of this 

 folding on the Jurassic than on the Cretaceous outcrop is probably to be 

 explained by the Jurassic rocks having already acquired a slight easterly 

 dip before the deposition of the Cretaceous. 



If we draw a line from the Dorset coast by way of the Cleveland Hills 

 to the Yorkshire coast, that line will pass along the outcrop of the Jurassic 

 rocks from the south coast to North Gloucestershire, and again near the 

 north-east coast. Between these points, however, the base of the Jurassic 

 swings away in a great arc to the eastward of the line, and in Lincolnshire 

 is more than 40 miles distant. Pari fassu with the divergence of the 

 outcrop from the line drawn through its extremities, the combined thick- 

 ness of the formations from the top of the Lias to the Oxford Clay 

 diminishes progressively towards East Anglia, and so far as I can determine 

 it appears that the lines joining points on the northern and on the 

 southern part of this arc, where the formations are of equal thickness, are 

 roughly parallel to the above-mentioned line. These facts render it 

 probable that the line drawn between the Dorset Coast and the Yorkshire 

 Coast is an isopachyte, or line of equal thickness in these formations, and 

 that other isopachytes corresponding to diminishing thicknesses follow 

 eastward until the line of zero thickness is reached under East Anglia. 

 Further, it is likely that the trend of the isopachytes gives a clue to the 

 general direction of the coastline. The subsequent arching of the Jurassic 

 rocks has, however, brought to the surface different isopachytes of these 

 formations, and along the crest of the arch the thicker zones have been 

 eroded away. If the Jurassic rocks had been upheaved along a N.N.E.- 

 S.S.W. line, it is probable that Lamplugh's generalization would have 

 held true for the whole extent of their outcrop. Owing, however, to the 

 later folding having occurred transversely to the isopachytes, the 

 generalization breaks down. It probably holds only when, as frequently 

 happens, lines of elevation tend to be parallel to pre-existing shore lines. 

 The effects of persistent axes of elevation or non-subsidence, as for instance 



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