part 1] . AKNTVEESAEY ADDEESS OF THE PEESTDENT. XC1 



the Jurassic strata are reduced to their northern minimum.-. 

 Throughout the Midlands, however, we can be sure that the Trias 

 occupies a trough parallel to, and in part coincident with, that of' 

 the Jurassic, and we have proof that the two systems behave very 

 similarly in their underground prolongation. The subsiding area 

 appears to have had nearly the same bounds during both periods, 

 but the trough of greatest depression (and therefore of thickest 

 accumulation) seems to have lain farther west in the earlier 

 period than in the later. The two masses of sediment are quite 

 different in character and mode of formation, but they have 

 followed the same regimen in obliterating the hollows of continued 

 subsidence. In both cases, also, the later reversing uplift was 

 most pronounced along the tracts in which the deposits were 

 respectively thickest ; that is to say, where the collecting troughs 

 were deepest. It is this differential recovery in the middle of the 

 basins that has brought about the prevalence of dips towards the 

 attenuated borders, which impressed Topley and led to his siDecu- 

 lation upon the cause. 



The Carboniferous rocks of Derbyshire and Yorkshire are cited. 

 by Topley as affording further examples of attenuation (again 

 south-eastwards) rapid enough to produce a perceptible dip. 

 Our knowledge of these rocks has been greatly extended lately by 

 borings that have traced their eastward prolongation beneath the 

 Permian and Trias, of which an excellent account has been given 

 by my colleague, Dr. W. Gibson. 1 It is now known that, south 

 of the Humber, the concealed Coal Measures combine with the 

 outcrop to form a tilted s} T ncline, which is fringed underground on 

 the east and south by the lower divisions of the Carboniferous 

 System, as it is on the west and north by the outcrops of these 

 divisions. The bottom of the LoAver Carboniferous rocks has not 

 been reached, nor has their eastward termination been proved ; 

 but the evidence points to their original attenuation in this direc- 

 tion, and to the probability that the Carboniferous wedge comes 

 to an end under the present Jurassic belt, partly from this cause, 

 but mainly through being shorn off by the Permian unconformity. 

 All the divisions thicken as they approach the region of the 

 Pennine uplift, and it appears certain that this great anticline has 

 arisen on the site of a former trough of depression and maximum 

 sedimentation. 



1 ' The Concealed Coalfield of Yorkshire & Nottinghamshire ' Mem. Geol. 

 Surv. 1913. 



