466 PHYSICAL GEOLOGY AND PALEONTOLOGY. 



the palaeontologist is with faunas as a whole. And he can only dis- 

 cover whence they came and whither they went in the ages of geolo- 

 gical history, by first knowing the physical history of given geological 

 deposits, and then considering the ways in which the changes in 

 neighbouring seas modified the life in the sea which the geological 

 stratum represents. 



We may illustrate these more complex studies by reference to the 

 Lower Tertiary strata. First, we find that, in tracing the Cretaceous 

 beds eastward, we come upon evidences of ancient shores and land 

 conditions. The Maestricht strata, though formed in shallower water, 

 may be no newer than the Norfolk chalk, and the chalk of the Danish 

 Archipelago and Scania may be essentially a prolongation of the same 

 deposit under conditions nearer to land ; but at Aix-la-Chapelle the 

 cretaceous sands, which may well be of chalk age, contain the rich 

 flora of an ancient land. Hence, then, without going further for other 

 evidence of a like kind, we may conclude that in certain parts of 

 Europe lacustrine and littoral strata were formed contemporaneous 

 with the English chalk. Now, if we believe these cretaceous islands 

 to have existed throughout the Chalk period, any change of level 

 which once more brought sediments into the seas where the chalk was 

 formed, must be supposed to have enlarged those islands, and have 

 made them a source for sands, such as constitute the oldest of English 

 Tertiary strata. 



The lowest of our Tertiary beds is named Thanet sands. JSTinety- 

 nine per cent, of the sand, according to Dr. Sorby, is granitic sand ; 

 the one per cent, is derived from chalk flints. Hence, we must pro- 

 bably look to find a range of crystalline rocks as the source of the 

 Thanet sands. We next notice the area over which the stratum is 

 distributed. It is not found west of a line drawn from St. Alban's 

 to Leatherhead. It is thinner on the northern outcrop of the London 

 basin than on the southern outcrop, where the thickness is 90 feet 

 at Canterbury, Erith 75 feet, Charlton 55 feet, Bank of England 40 

 feet, Battersea 1 7 feet, Leatherhead 3 feet. If we cross the Channel 

 the thickness is 90 feet at Calais, and in Belgium the stratum is well 

 represented by the Landanien inferieur, which becomes somewhat 

 coarser in its eastward extension. There is hence a strong probability 

 that, as the sand was derived from a granitic source, it would be suc- 

 ceeded, farthest from the source in horizontal direction, by a clay. If 

 the antecedent circumstances of the Tertiary rocks justify us in looking 

 to Central Europe for the rocks which yielded the Thanet sands, then 

 we should expect, when the sands thin away westward, to find that 

 the clay which replaces them would be of the same age, that is to 

 say, that a clay would rest upon the chalk in the western part of the 

 London basin and in the Hampshire basin. To the Thanet sands 

 succeed the Woolwich and Beading beds ; they are the Landanien 

 supe"rieure of Belgium. They are marine sands in East Kent, but 

 from Rochester westward give evidence of nearness to land in the 

 mixture of fresh-water shells like Neritina, with estuarine types like 

 Melania and Cerithium, with marine genera like Ostrea ; and at Lewis- 



