( ( The ( past ( paleomagnetic history ) ) ) ( ( ( ( is ( well established ) ) , ) ( ( in that ) ( ( during ( ( discrete intervals ) ( of ( ( the ( earth 's ) ) past ) ) ) ) ( , ( ( the ( magnetic poles ) ) ( have ( switched polarity ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (JJ past) (JJ paleomagnetic) (NN history)) (VP (VBZ is) (ADJP (RB well) (VBN established)) (, ,) (SBAR (WHPP (IN in) (WHNP (WDT that))) (S (PP (IN during) (NP (NP (JJ discrete) (NNS intervals)) (PP (IN of) (NP (NP (DT the) (NN earth) (POS 's)) (NN past))))) (, ,) (NP (DT the) (JJ magnetic) (NNS poles)) (VP (VBP have) (VP (VBN switched) (ADVP (RB polarity))))))) (. .)))
The past paleomagnetic history is well established, in that during discrete intervals of the earth's past, the magnetic poles have switched polarity.
(ROOT (S (NP (NP (DT The) (NN earth) (POS 's)) (JJ magnetic) (NNS poles)) (VP (VBP have) (VP (VBN switched) (NP (NNS places)) (ADVP (RB repeatedly)) (PP (IN in) (NP (DT the) (NN past))))) (. .)))