XVII.] GEOLOGY OF THE THAMES BASIN. 293 



which existing sponges of similar kinds always contain. 

 On the other hand, the chalk contains flints, of which no 

 trace is found in the ooze. On the whole, it is probable, 

 that these flints represent the siliceous organisms which 

 were contained in the Cretaceous ooze, when it was de 

 posited, but which have been dissolved up by percolating 

 water, and re-deposited in the shape of amorphous silex ; 

 just as the diatoms in the Bihn beds have been dissolved 

 and re-deposited as opal. 



The lowermost beds of the chalk rest upon sandy de- 

 posits, which are termed the Upper Greensaiid. Some of 

 these sandy beds are not the mere mechanical detritus of 

 siliceous rocks, but contain numerous greenish grains of 

 definite and well-marked forms. They are. in fact, the casts 

 of the internal cavities of foraminiferal shells in a compound 

 of silica, iron, and clay, which is known as silicate of iron 

 and aluminium. This substance has been deposited in the 

 cavities of the shells, the calcareous matter of which has 

 been subsequently dissolved away, and has left the hard 

 cast behind. A similar sandy deposit is taking place off 

 the eastern coast of the United States, upon the Agulhas 

 bank, near the Cape of Good Hope, and elsewhere, at the 

 present time, in depths varying from 100 to 700 fathoms. 

 A tolerably deep sea, therefore, covered the area now 

 occupied by the chalk, not only during the time which 

 was occupied by the deposition of the chalk, but ante- 

 cedently to that period. 



But we can carry the evidence of the existence of a 

 sea covering the western part of the Thames basin, to a 

 still more remote period. The organic remains which are 

 found in the lower cretaceous strata and in the oolitic and 

 liassic beds, which underlie the chalk, and are exposed in 

 this region, are chiefly those of marine animals. In the 



