360 SPECIAL GEOLOGY. 



state from depths not previously explored. The crag fauna 

 has therefore been found to approach much nearer to the 

 recent fauna of the Northern British and Mediterranean 

 seas than had been imagined. The analogy of the whole 

 group of testacea to the European type is very marked, 

 whether we refer to the large development of certain genera 

 in number of species, or to their size, or to the suppression 

 or feeble representation of others. The indication also 

 afforded by the entire fauna of a climate not much warmer 

 than that now prevailing in corresponding latitudes, prepares 

 us to believe that they are not of higher antiquity than the 

 older Pliocene era.* 



The deposits which occupy the basin of London, and the 

 valley of the Thames, fill up a depression of the chalk, which, 

 commencing from the North Downs on the south, termi- 

 nate on the east with the Isle of Sheppey, on the west with 

 the sea, and are bounded on the north-west by the re-appear- 

 ance of the chalk in the hills of Berkshire, Wiltshire, Oxford- 

 shire, Buckinghamshire, and Hertfordshire. These accumu- 

 lations were once divided into three distinct groups, the 

 Bagshot sand, London clay, and Plastic clay, but, by later 

 observers, they have been reduced to two; the first consisting 

 of those superficial beds which compose the sands of High- 

 gate, Hampstead, Pinchley, Bagshot Heaths, and other are- 

 naceous deposits in the vicinity of the metropolis ; the 

 second comprises those underlying deposits of shingle, clay, 

 loam, and sand, w r hich constitute the London clay formation. 

 The upper division of the latter is 

 composed of indurated clays, of 

 various tints, chiefly bluish or brown, 

 and frequently containing ovate 

 nodules of argillaceous limestone, 

 divided by fissures, filled with veins 

 of crystallised carbonate of lime or 

 sulphate of barytes, radiating from 

 FIG. 259. Septarium. the centre to the circumference, 



,'V produced by the cracking of the 

 clay when drying, and by the subsequent infiltration of the 

 mineral substance into the cavities thus occasioned (fig. 259). 



Sir C. Lyell, Manual of Elementary Geology, 1851. 



