A. J. Jukes-Browne — The Cenomanian Overlap. 495 



sporadically it does not seem to fill a cavity, or to have replaced 

 a less stable species of felspar, which indeed, as the rock is in good 

 preservation, is hardly probable. So I think the mineral must have 

 been present as a constituent when the rock was molten, and can 

 only suppose that the latter, on its ascent to the surface, caught 

 up some intervening limestone, and the pressure sufficed, as in 

 a case of contact metamorphism, to prevent the dissociation of the 

 carbonate, which here and there retained its identity in the viscid 

 mass.'^ This occurrence in such a rock as basalt, is, I think, rather 

 unusual. 



IV. — The Cenomanian Overlap. 

 By A. J. JuxES - Browne, B.A., F.G.S. 



THAT the Upper Cretaceous deposits overlap those of the Lower 

 Cretaceous series, and extend over much larger areas throughout 

 the whole of western and central Europe, are facts which have long 

 been familiar to all European geologists. There has, however, been 

 a tendency, especially in France, to imagine that the subsidence to 

 which the overlap is due took place at one particular epoch, namely, 

 the Cenomanian, and that this subsidence was so rapid and profound 

 that it caused the Upper Cretaceous Series to be sharply marked 

 off from the subjacent strata by the so-called ' Cenomanian 

 transgression.' 



Moreover, some French geologists so magnified the importance of 

 this transgression that they regarded it as a criterion for deciding 

 whether a given stage or zone should be classified as Lower or 

 Upper Cretaceous. Thoir views have doubtless had much influence 

 upon Continental opinion, and are largely responsible for the classi- 

 fication which places the ' Albian ' in the Lower Cretaceous Series, 

 and for the uncertainty as to whether the zone of Am. (Schloenhachia) 

 rostratiis should be grouped with the Albian or the Cenomanian. 



In England there has never been any such doubt : we have 

 always included the Gault (Albian) in the Upper Cretaceous Series, 

 and have regarded it as partaking in the transgression of that series 

 beyond the limits of the Vectian or Lower Greensand. 



The French view dates from the time of D'Orbigny, who wrote 

 in 1840 as follows :— " From the time of the craie chloritee (which 

 he afterwards called Cenomanien) we see that the whole aspect 

 of the Cretaceous seas was changed. ... At about this epoch 

 the extension of the seas both in France and throughout Europe 

 was at least double that which they had at the time of their first 

 invasion of the region in the Neocomian period." This generalization 

 may have been in accordance with the knowledge of D'Orbigny's 

 time, but is not maintainable in the light of more recent investigations. 

 It should be remembered that when D'Orbigny thus wrote, and for 

 many years afterwards, he believed that there was a marked dis- 

 cordance between the Albian and the Cenomanian deposits, and that 



1 For cases of the imperfect digestion of a carbonate in an igneous mass, see 

 J. Parkinson, QJ.G.S., Ivii (1901), p. 198, and A. K. Coomaraswamy, id., Iviii 

 (1902), p. 399. 



