CRETACEOUS AGE. 



before, that of the first, but that of the fourth Cretaceous period 

 (20). 



410. The interpretation of this fact is evidently a change of 

 relative level of the land along this border during the first four 

 periods of the Cretaceous age. In the first, second, and third 

 periods the border was subject to a gradually increasing depression, 

 which after each period caused the shore of the sea to advance 

 outwards, the shore of the second period (18) being outside that 

 of the first (17), that of the third (19) outside that of the second 

 (18), and in fine that of the fourth (20) being outside that of the 

 third (19). The consequence of this is, that the fourth stage of 

 the Cretaceous formation along this part of the coast lies over 

 and conceals the previous deposits forming the first (17), second 

 (18), and third (19) stages. But after the fourth period the 

 relative levels must have undergone a contrary change, causing 

 the borders of the sea to have retired inwards, or eastwards, from 

 period to period, so that between the fourth and fifth period a 

 band of deposit of the fourth stage (20) remained uncovered, and 

 after the next period another (21), and so on to the end of the 

 Cretaceous age. 



411. The irregularity indicated above, as having taken 

 place on the western side of the Paris basin, might have been 

 expected to have continued across the channel, and to reappear 

 along the English, border of the same basin. Such, however, is 

 not found to be the case ; for, as indicated in the map, the suc- 

 cessive stages of the Cretaceous period from the first (17) to the 

 last appear along the northern border of the basin from Dorset- 

 shire to Yorkshire, succeeding each other in the same regular 

 order as in the southern border in France. It would, therefore, 

 appear that the undulation which produced the irregularity 

 observable along the western border adjoining Brittany must 

 have been littoral ; that is, in a direction at right angles to the 

 N.W. and S.E. borders of the basin. 



412. An examination of the Pyrenean basin discloses a singu- 

 lar state of things in accordance with that observed in the western 

 border of the Parisian basin. The first three stages of the Cre- 

 taceous formation are altogether absent from the Pyrenean basin, 

 which only received the deposits of the four last periods. M. 

 D'Orbigny has inferred from this that during the first three 

 periods of the Cretaceous age, the ground upon which the Pyre- 

 nean basin is placed was raised above the level of the sea, but 

 that the convulsion which closed the third Cretaceous period 

 caused it again to sink so as to be covered by the sea, and that it 

 thus remained submerged during the last four Cretaceous periods. 

 The border retiring from period to period is indicated on the map, 



103 



