118 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



not contemporary with some portion of either the Portland stone or 

 lower greensand. 



It is evident that the Wealden fossils enumerated in the list 

 given in a preceding paragraph, together with those of the beds of 

 Yorkshire, Sutherlandshire, &c., must be regarded as the remains 

 of animals and vegetables which inhabited the land, and t:ie fresh and 

 braclsish waters of the Jurassic period. Hence it might be supposed, 

 that the Portland stone (under the assumption already made, that 

 it is the newest of the oolites) was more likely than the lower 

 greensand to be the contemporary of the portion of the Wealden 

 now under consideration. But as there is everywhere a sharply de- 

 fined boundary between the Portland stone and the Purbeck beds 

 in contact with it, and we have the testimony of the "dirt-beds," to 

 prove that a considerable portion of the surface of the former became 

 dry land very soon after the deposition of the latter commenced, 

 the two cannot have accumulated contemporaneously. Neverthe- 

 less, if the view which 1 have taken of the relations of the Wealden 

 and Jurassic series be the correct one, strata belonging to the former, 

 although not yet recognized, must have been deposited in the lakes, 

 rivers and estuaries of the Portland-stone sera, as well as during the 

 periods consumed in the formation of the preceding members of the 

 system. 



I shall next consider to what extent the lower greensand can be 

 looked upon as contemporary with the Wealden strata which it 

 rests upon. Before doing so, however, it will be necessary to advert 

 to the proportions of land and water which existed in the European 

 region of the globe during the Jurassic epoch. On looking at a 

 geologically-coloured map of this area, it will be seen that a very 

 large portion of its surface is covered by marine Jurassic rocks, and 

 when to this is added the amount concealed by the superposition 

 of newer strata and that removed by denudation, it is clear that but 

 little of the space could have been occupied by land ; and conse- 

 quently the rivers must have been proportionately diminutive. 



We have here a reason for the rarity of Wealden beds through- 

 out the oolites ; but on the elevation of the Portland stone, coupled 

 with the upheaval of mountain masses in the interior, which doubt- 

 less accompanied the change of level*, the state of matters was 

 changed, and the rivers, their volumes augmented in proportion to 

 the increased area of their hydrographical basins, now poured down 

 correspondentiy great supplies of sediment. Thus, I think, is ex- 

 plained the enormous development of the Wealden deposits at the 

 termination of the Jurassic epoch. 



The number of identical species in the Portland stone and lower 

 greensand of Central and Western Europe is so small, that, in taking 

 for granted the absence of intermediate beds, it seems proper to offer 

 some observations on the means by which this almost total change 

 of organic beings may have been tfftcted. There appears to me to 



* Mr. Darwin on Volcanic Phaeuomena in South America, Trans. Geol. Soc. 

 2nd Ser., vol. v. p. 601 et seq. 



