290 PHYSIOGRAPHY. [chap. 



the Reculvers, and in Pegwell Bay, in the Isle of Thanet, 

 where they consist of sands containing marine shells. 



All the strata noticed above are classed together in one 

 great group known as the Tertiary or Cainozoic ^ series. The 

 former name refers to the fact that geologists have been led, 

 in their study of the earth's crust, to recognize three great 

 groups of rocks, of which, those hitherto described in this 

 chapter, represent the uppermost or third, reckoning from 

 below upwards. The Tertiary series, as developed in the 

 London basin, comprises all rocks, from the Thanet sands 

 below, to the Bagshot beds above. Over this Tertiary 

 group, come the glacial-drifts, and river-gravels, which are 

 sometimes grouped together as a fourth set of deposits, and 

 are consequently known as the Quaternary series, while by 

 others they are called Post-tertiary formations. While the 

 Tertiary beds are succeeded above by the quaternary series, 

 they are followed below by another great group, known as 

 the Secondary or Mesozoic^ series, of which the uppermost 

 member is the well-known chalk. 



The pale-green tint on the map (Plate V.) covers a large 

 area occupied superficially by the Chalk; while the rela- 

 tion of this rock to the overlying Tertiaries may be seen 

 in the coloured section in the same plate,^ and has already 

 been shown diagrammatically in Fig. ii (p. 31). The 

 ground formed by the chalk usually consists, when not 

 covered with drift, of gently undulating downs, carpeted 

 with soft turf. Good examples of chalk scenery may be 

 seen in the North Downs of Surrey and Kent, and in 



^ Cainozoic, from Kaivos, kainos, recent ; C^ov, zoon, an organism. 



* Mesozoic, from fieaos, mesos, middle. 



' Tliis section, drawn by Mr. Wliitalcer, of the Geological Survey, is 

 supposed to be taken along the line, A B, on the map. The vertical 

 scale of the section is about tvi'elve times greater than the horizontal 

 scale, and the latter is evidently larger than that of the map. 



J 



