160 Marine Microbiology 



North Geographic Pole 

 South Magnetic Pole 



South Geographic Pole 

 North Magnetic Pole 



Fig. 1. The earth's magnetic field. 



loo Bridge in London (3). Chapman and Bartels (3) describe 

 a number of subsequent experiments performed by the British 

 Admiralty showing the effects of the vertical component of the 

 earth's field in producing potential differences. More recently 

 Bauer and Norton (2) have postulated a similar situation to 

 exist in some rivers. Fossil shells recovered from one side of a 

 river bank during a field study were found to be badly decalcified. 

 Shells found in the opposite bank in the same formation were well 

 preserved. They concluded that a physical-chemical process 

 was taking place involving induced electrical currents resulting 

 from the conducting river water flowing across the earth's mag- 



