242 Woolwich and 



looked upon as a portion of the Woolwich and Reading 

 series, which partly consists of a few saltwater beds, 

 interstratified with a preponderance of fresh-water 

 deposits. Excepting that the Thanet Sand is alto- 

 gether marine, it is possible that it might have con- 

 tinued still to be classed simply as one of the minor 

 marine portions of the Woolwich and Heading series. 



The Woolwich and Reading beds, formerly called 

 the Plastic Clay (Argile plastique of the Paris basin), 

 overlie the Thanet Sand, and rest directly on the Chalk, 

 when, as in the greater part of the London basin, and 

 in Hampshire and the Isle of Wight, the Thanet Sand 

 is absent. They may be broadly described as consisting 

 of many wedge-shaped interstratitications of mottled 

 clays, light-grey sands, and pebble-beds, made of chalk 

 flints, which are sometimes loose and gravelly, and 

 sometimes hardened into conglomerates. From west to 

 east the strata vary from 15 to 90 feet thick in the 

 London basin. In the Hampshire basin they are still 

 less developed (fig. 47), and the whole consists of 

 mingled marine, estuarine, and often of purely fresh- 

 water strata, marking the first obvious signs of the 

 influx of a great river, formed by the drainage of a con- 

 tinent, the result of the upheaval above the sea of 

 large areas of Chalk and other older rocks in what is 

 now Britain and the nearest parts of France. There 

 can be no doubt, however, that the Thanet Sands are 

 the result of the same set of conditions, only they were 

 deposited further from shore in a comparatively open sea. 



More than 100 species of fossils are known in the 

 Woolwich and Reading strata, including an herbivorous 

 mammal of the genus Coryphodon, allied to the modern 

 tapirs of South America, which live on the banks of the 

 Amazons and other great rivers, also the bones of a bird, 



