CHAP, X.] 



THE EARTH'S AGE. 



211 



Limestone full of characteristic fossils, at a depth of only 800 

 feet. Here we have an enormous gap, showing that none of 

 earlier Secondary or late Paleeozoic formations extend to this 

 part of England, unless indeed they had been all once elevated 

 and entirely swept aAvay by denudation. i 



But if we consider how such deposits are now forming, we 

 shall find that the thinning out of the beds of each formation, 

 and their restriction to irregular bands and patches, is exactly 

 what we should expect. The enormous quantity of sediment 

 continually poured into the sea by rivers, gradually subsides to 

 the bottom as soon as the motion of the water is checked. All 

 the heavier material must be deposited near the shore or in 

 those areas over which it is first spread by the tides or currents 

 of the ocean ; while only the very fine mud and clay is carried 

 out to considerable distances. Thus all stratified deposits will 

 form most quickly near the shores, and will thin out rapidly at 

 greater distances, little or none being formed in the depths of 

 the great oceans. This important fact was demonstrated by 

 the specimens of sea-bottom examined daring the voyage of 

 the Challenger, all the shore deposits " being usually confined 

 within a distance of 100 or 150 miles from the coast, while the 

 ' ' deep-sea deposits " are either purely organic, being formed of 

 the calcareous or siliceous skeletons of globigerinse, radiolarians, 

 and diatomacese, or are clays formed of undissolved portions of 



1 The following statement of the depths at which the Palaeozoic forma- 

 tions have been reached in various localities in and roimd London was 

 given by Mr. H. B. Woodward in his address to the Norwich Geological 

 Society in 1879: — 



Deep Wells through the Tertiary and Cretaceous Formations. 



Harwich at 1,022 feet reached Carboniferous Eock. 



Kentish Town „ 1,114 „ „ Old Eed Sandstone. 



Tottenham Court Road „ 1,064 ,, ,, Devonian. 



Blackwall „ 1,004 „ Devonianor Old Red Sandstone, 



Ware „ 800 „ „ Silurian (Wenlock Shale). 



We thus find that over a wide area, extending from London to Ware and 

 Harwich, the whole of the formations from the Oolite to the Permian are 

 wanting, the Cretaceous resting on the Carboniferous or older Palaeozoic 

 rocks ; and the same deficiency extends across to Belgium, where the 

 Tertiary beds are found resting on Carboniferous at a depth of less than 

 400 feet. 



p 2 



