Missing Chapters of Geological History. 23 



contain remains of land plants. Everywhere the Portland 

 rock is fragmentary. Tlie Stonesfield slate is remarkable for 

 its mammalian bones, but is very local. And then, lastly, the 

 series terminates with the Purbeck and Wealden rocks, a 

 deposit, for the most part from fresh water, thrown down at 

 the commencement of the interval that elapsed between the 

 middle and upper Secondary formations. 



When in the lowest beds of the lower Greensand we find 

 deposits not always unconformable in their stratification with 

 the upper Oolites, but separated absolutely in the character of 

 its fossils, we recognize that we have passed another of the 

 great breaks, in succession, on which much of the sharpness of 

 geological definitions may be said to depend. The contents of 

 the Purbeck beds are, for the most part, unknown in the 

 Oolites ; and it has been suggested, with some show of proba- 

 bility, that while the true Wealden are undoubtedly delta 

 deposits, these are lacustrine. 



But there must have been a long period, not marked with 

 any considerable disruptions or cataclysmic disturbances, be- 

 tween the last Oohtic and the first Cretaceous marine deposit. 

 All had had time to change, and the accumulation of some 

 2000 feet of fresh-water mud and sand must have needed time, 

 that may elsewhere have been employed in removing by 

 denudation rocks already at the surface. 



Afterwards we have the Cretaceous series wonderfully rich 

 in fossils, chiefly in the upper part (the white chalk), but not 

 wanting in any important department. Here there is evi- 

 dence of a break of some importance. Between the lower and 

 the upper Greensand, out of 280 species only fifty-one pass 

 upwards from the one to the other, through the Gault. A 

 real stratigraphical break also exists, the Gault lying uncon- 

 formably over the lower Greensand, at various points round the 

 Weald and elsewhere. There is no other important break 

 till above the upper chalk, where we meet with that most im- 

 portant, most widely recognized, and most extensive of all — 

 the great line of demarkation between Secondary and Tertiary. 

 Only one species of Terebratula (T. caput serpentis), and a 

 few Foraminifera, survived during that long period between 

 the close of the Cretaceous and the commencement of the 

 Eocene epochs. 



Professor Ramsay, in his addresses, does not continue the 

 discussion beyond the close of the Secondary period. In 

 England, the indication of breaks within the Tertiary period 

 are several and well marked, but they have not been worked out 

 systematically, with due reference at once to stratification and 

 change of organisms. Certainly, such a break occurred after 

 the close of the Eocene period, the whole great series of Miocene 



