Mr. Cowham's Address 281 



directed to the map, and the flat and hilly dis- 

 tricts shown on the map are seen to change from 

 one to the other, i.e. from flat to hilly, with the 

 changes in rock - formation already noticed on the 

 plan, e.g. the low-lying Deptford area with the river 

 alluvium, the flat area to Croydon with the London 

 Clay, the hills of Shirley and Addington with the 

 sands and gravels of the Lower London Tertiaries, 

 and the elevated and undulating area — North Downs 

 at' Purley — with the Chalk. 



The comparison indicated above is continued south- 

 wards from Caterham to Tilburstow Hill — a distance 

 of about four miles, during which not less than four 

 well - defined and very difl'erent rock forms come to 

 the surface, each form in turn giving a new aspect 

 to the surface appearance, as, for instance, the long 

 flat region of the Gault just beyond the Chalk, and 

 the rapidly-rising area beyond the Gault due to the 

 appearance of the sands and ironstone bands of the 

 Folkestone and Hythe beds. 



The preparation is completed by a brief reference 

 to the behaviour of a few of these rocks under the 

 action of frost, rain, and running water. For example, 

 the tendency of the London Clay to flatten itself into 

 an extended plain; the protection afforded by the 

 accumulation of stones on the summits of gravel 

 deposits, thus maintaining the elevated character of 

 all such ridges ; and the tendency of all Chalk areas to 

 weather into long sweeping curves; and lastly, the pre- 

 sence of iron bands in sand deposits tending to pre- 

 serve all such sandy ridges from being further lowered. 



After preparation such as I have briefly described, 

 the students with their maps, plans, and sections in 



