Was it a sudden decision on his part, or had he already made up his mind when he parted from me a few hours earlier? 
( ( ( ( ( ( Was it ) ( ( a ( sudden decision ) ) ( on ( his part ) ) ) ) , ) or ) ( ( had he ) ( already ( made ( up ( ( his mind ) ( when ( he ( parted ( ( from me ) ( ( a ( few hours ) ) earlier ) ) ) ) ) ) ) ) ) ) ) ? )
(ROOT (SQ (SQ (VBD Was) (NP (PRP it)) (NP (NP (DT a) (JJ sudden) (NN decision)) (PP (IN on) (NP (PRP$ his) (NN part))))) (, ,) (CC or) (SQ (VBD had) (NP (PRP he)) (VP (ADVP (RB already)) (VBN made) (PP (IN up) (NP (NP (PRP$ his) (NN mind)) (SBAR (WHADVP (WRB when)) (S (NP (PRP he)) (VP (VBD parted) (PP (IN from) (NP (PRP me)) (ADVP (NP (DT a) (JJ few) (NNS hours)) (RBR earlier)))))))))) (. ?)))
He left in the spur of the moment whilst we were talking, which was quite rude. 
( He ( ( ( left ( in ( ( the spur ) ( of ( the moment ) ) ) ) ) ( ( ( whilst ( we ( were talking ) ) ) , ) ( which ( was ( quite rude ) ) ) ) ) . ) )
(ROOT (S (NP (PRP He)) (VP (VBD left) (PP (IN in) (NP (NP (DT the) (NN spur)) (PP (IN of) (NP (DT the) (NN moment))))) (SBAR (SBAR (IN whilst) (S (NP (PRP we)) (VP (VBD were) (VP (VBG talking))))) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBD was) (ADJP (RB quite) (JJ rude))))))) (. .)))