AGE OF MAMMALS. 277 



once by the porous mass, so that there are no rills run- 

 ning down the sides of the cones to wear them away. 

 The highest of the volcanoes is Mont Dor, in Auvergne, 

 which was several thousand feet above the surrounding 

 platform, and retains now the shape of a flattened cone, 

 broken on its summit into several rocky peaks, probably 

 from occasional earthquakes in ages long gone by, and 

 the continued influence of rain through not merely many 

 centuries, but many ages. 



387. Basins. From the disturbances which occurred 

 in the Tertiary period, there are strata found in some lo- 

 calities so arranged in flexures as to form what are called 

 basins. Among the most noted of these are the basins 

 of London and Paris. In Fig. 167 is represented, in a 



Fig. 167. 



rude way, that of London. At 4 we have the chalk ly- 

 ing upon green sand, 5. Upon the chalk lies the plastic 

 clay, so called because in France, where it is less mixed 

 with other materials, it is extensively used in pottery. 

 Here there are in it beds of flint and pebbles alternating 

 with sands and clay. Upon it lies the London clay, 2, 

 upon which the city of London stands. It varies from 

 two to six hundred feet in thickness, is of a dark color, 

 tough in texture, and having mixed with it here and 

 there earth of a green color, white sand, and masses of 

 clayey limestone. At 6 is the River Thames. At 1, 1 

 are caps of marine sand, which are found on many of the 

 hills in the valley of the Thames. This sand probably, 

 when first deposited, formed a continuous bed over the 

 region, but has been excavated with a part of the Lon- 

 don clay, an example of the denuding agency of water. 



