XVII.] GEOLOGY OF THE THAMES BASIN. 281 



time when the river spread out, at certain parts of its course, 

 into wide lake-like areas, and quietly deposited mud and 

 sand at the bottom of these sheets of water. The strips of 

 alluvtian (p. 142) deposited by the river along its margins, in 

 comparatively modern times, are also indicated on the map. 

 Fig. 81. This marshy land, bordering the Thames and its 

 tributaries, spreads out below London into wide flats; and 

 it occasionally contains shells, bones and other organic 

 remains. Of the vegetable relics, which are found in some 

 of the marshes, and indicate the site of an ancient forest, no 

 mention need be made here, since they have already been 

 noticed at p. 212. 



Fossils are not confined, however, to the comparatively 



tiG. i,-i.—Cyreiin (Corbicula) jJuminalis. 



modern mud which forms the alluvium of the Thames ; but 

 they are also found, more or less abundantly, in the older 

 superficial deposits, such as the gravels and brick-earths. 

 Thus, they are especially abundant in the brick-earth which 

 is worked at Erith and Crayford in Kent, and at Ilford and 

 Grays in Essex; while, above London, they have been found 

 in the sands and loam of Brentford. Many of these fossils 

 are land and fresh-water shells, which once lived in the 

 river and on its banks ; and are, for the most part, not 

 different from those living in the locality at the present day. 

 Some few of the shell-fish, however, have long ceased to 

 dwell in the rivers of this country, thouglr still found in 



