00267nam a2200109 a 450000500170000002000150001708200190003210000160005124500440006726000310011130000150014220251203091722.0  a0852744420  a538.7,bD, JAC  aJacobs J.A.  aReversals of the earth's magnetic field  aBristolbAdam Hilgerc1984  a230p, 22cm