218 



ISLAND LIFE 



PART I 



seen ia the case of the Secondary rocks of England), and a 

 corresponding section showing each bed dipping beneath 

 its predecessor, we are apt to imagine that beneath the 

 uppermost bed we should everywhere find all the others in 

 succession like the coats of an onion. But this is far 

 from being the case, and a remarkable proof of the narrow 

 limitation of these formations has been recently obtained 

 by a boring at Ware through the Chalk and Gault Clay, 

 which latter immediately rests on the Upper Silurian 

 Wenlock Limestone full of characteristic fossils, at a depth 

 of only 800 feet. Here we have an enormous gap, show- 

 ing that none of earlier Secondary or late Palaeozoic 

 formations extend to this part of England, unless indeed 

 they had been all once elevated and entirely swept away 

 by denudation.^ 



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 



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

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

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

 Society in 1879 :— 



Beep Wells through the Tertiary and Cretaceous Forrnations. 



Harwich at 1, 022 feet reached Carboniferous Rock. 



Kentish Town 1,114 Old Red Sandstone. 



Tottenham Court Road,, 1,064 Devonian. 



Blackwall 1,004 ,, ,, Devonian or 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 Palseozoic 

 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. 



