part 1] anniversary address or the president. xciii 



Persistence of Outcrop in relation to Thickness. 



The corollary is, that the present outcrops -of the formations 

 involved in the structure coincide more or less closely with the 

 tracts in which the formations severally attained their greatest 

 original thickness. This appears to be a general rule wherever 

 any such formation rises wholly above present sea-level, but it may 

 not apply where the formation lies wholly or mainly below that 

 level. The recognition of this relationship in the Jurassic out- 

 crops of the Midlands was incidentally stated by Hull in his paper 

 .Of 1859 (Q. J. G-. S. vol. xvi, 1860, p. 77) in the following terms :— 



' It is scarcely possible to estimate the advantage, both to science and 

 to civilization, which has resulted from the present configuration of the 

 Mesozoic rocks of England. Keeping in mind the tendency which they 

 exhibit to thin away to the south-east, it is evident that, with their present 

 strike and inclination, they are presented to us in succession along lines of 

 fullest development. In order to estimate this more fully, we have only to 

 reflect how dwarfish would have been the ascending series of formations, had 

 they been upheaved along an axis coinciding with the present escarpment of 

 the Chalk from Reading to the German Ocean.' 



That Hull's statement applies with equal force to the Trias, 

 Lias, and all the Oolites, not only of the Midlands but of the North 

 of England also, is evident from the sections that we have been con- 

 sidering (figs. 3 & 4, facing p. lxxx) ; and needs no further comment. 

 The same condition is even more strikingly apparent in the Lower 

 Cretaceous succession, in which the great Wealden Series, as we 

 have seen, dwindles to nothingness away from its outcrop ; and the 

 marine Lower Greensands have been found to diminish almost as 

 rapidly underground wherever the boring-test has been applied. 

 The Chalk affords a further example, although under different 

 circumstances. In this case the thickest part of the formation is. 

 on the east below sea-level, and the mass becomes thinner in every 

 subdivision when traced westwards up to the present escarpment. 

 That it continued to lose thickness in this direction may be safely 

 assumed ; and, although we cannot be certain of its original limits, 

 there are indications, in the composition of the lower portion s r 

 that they at least, and probably the whole formation, wedged out 

 against the ancient uplift of the western country. Therefore the 

 existing outcrop represents the thickest part of the original mass 

 now above sea-level. The same argument will hold good, I 

 believe, in every particular, for all the Tertiary strata of the south 



