1872.] EA.MSAT RIVER-COtrESES OF ENGLAND AND WALES. 153 



receded eastward; and as it did this the area of drainage contracted. 

 By and by the outcropping edges of OoHtic strata became exposed, 

 and a second and later escarpment began to be formed ; but the 

 escarpment of the Chalk being more easily wasted than that of the 

 Oolite, its recession was more rapid. 



All this time the Thames was cutting a valley for itself in the 

 Chalk ; and by and by, when the escarpment had receded to a cer- 

 tain point, its base was lower than the edge of the Oolitic escarp- 

 ment that then as now overlooked the valley of the Severn ; only at 

 that time the vaUey was narrower. While this point was being 

 reached, the Thames by degrees was joined by the waters that • 

 drained part of the surface of the long eastward slope of the Oolitic 

 strata, the western escarpment of which was still receding ; and thus 

 was brought about what at first sight seems the unnatural breaking 

 of the river through the high escarpment of the Chalk between 

 "Wallingford and Reading, This, also, is the reason why the so- 

 called sources of the Thames, the Seven SjDrings and others, rise so 

 close to the great escarpment of the Inferior Oolite east of Gloucester 

 and Cheltenham. But the sources of the river now are not more 

 stationary than those that preceded them. The escarpments both of 

 Chalk and Oohte are still slowly changing and receding eastward, 

 and as that of the Oolite recedes, the area of drainage wiU diminish 

 and the Thames decrease in volume. This is a geological fact, how- 

 ever distant it may appear to persons unaccustomed to deal with 

 geological time. 



The same kind of argument is applicable to the Ouse, the Nen, 

 the Welland, the Glen, and the Witham, rivers flowing into the 

 Wash, all of which rise either on or close to the escarpment of the 

 Oolites, between the country near Buckingham and that east of 

 Grantham, on rocks which were once covered by the Chalk. 



With minor differences, the same general theory equally applies 

 to all the rivers that run into the Humber. I beheve the early 

 course of the Trent was established at a time when, to say the least, 

 the Lias and Oolites overspread aU the undulating plains of ISTew 

 Red Marl and Sandstone of the centre of England, and passed out 

 to what is now the sea, beyond the estuaries of the Mersey and the 

 Dee. A high-lying anticlinal Kne threw off those strata, with low 

 dips to the east and west ; and, after much denudation, the large 

 outlier of Lias between Market Drayton and Whitchurch, in Shrop- 

 shire, is one of the western results. Down the eastern slopes the 

 Trent began to run across an inclined plain of Oolitic strata. 

 Through long ages of waste and decay the Lias and Oolites were 

 washed away from all these midland districts, and the long escarp- 

 ment formed of these strata now lies well to the east, overlooking 

 the broad valley of New Red Marl through which the river flows. 



The most important tributary of the Trent is the Derwent, a tri- 

 butary of which is the Wye of Derbyshire. The geological history 

 of the latter river is very instructive. It runs right across part of the 

 central watershed of England formed by the great boss of the Car- 

 boniferous Limestone of Derbyshire. This course, at first sight, 



