ITS SUBSTRUCTURE. 59 



series clearly lead to the conclusion that the material forming the 

 beds was deposited under both marine and estuarine conditions, 

 the latter prevailing in the eastern area, where, at Charlton, near 

 Woolwich, river and estuarine shells may be found in great 

 abundance ; so abundant, indeed, are they, that some beds are 

 almost entirely composed of them. 



Although the Thanet Sands are purely marine in origin, they 

 were deposited in a small sea, and have a very limited extension. 

 Thickest in the east (in the Isle of Thanet — whence their name), 

 they gradually thin out to the westward, and do not seem to 

 extend further in that direction than to the neighbourhood of 

 Ealing. There are not many exposures of these beds on the 

 north of the Thames ; but on its southern bank at Charlton, Erith, 

 and Upnor, they are well seen showing, by artificial sections — the 

 sands being extensively worked for iron-casting purposes — 

 exposures of buff-coloured loamy sands 70 or 80 feet thick. 



The very important Chalk formation, which has a maximum 

 aggregate thickness of upwards of 1,000 feet, and an extension in 

 England from Dorsetshire to the Forelands and from the South 

 Coast to Flamborough Head, is of such a remarkable character 

 that much could be said about its origin, its composition, and its 

 structure. It may, however, suffice for these pages to state that 

 it is a great accumulation of calcarious matter formed by minute 

 animals that lived in a deep and open sea, such as the present 

 Atlantic Ocean, in which it has been ascertained that a similar 

 deposit is now being formed. 



This Chalk underlying the Tertiary beds below Hampstead 

 Hill, as will be seen from the section (Plate IX.), is not only con- 

 tinuous with the chalk of Watford and Hertfordshire on the 

 north, but also with the chalk of Croydon and Surrey on the 

 south. Dipping to the south, it extends quite under the valley of 

 the Thames, and then, gently rising, comes to the surface at 



