Tommy heard himself saying idiotically: "The gorse won't be there after all these years." And Julius replied solemnly: "I guess you're right." Tommy suddenly pointed with a shaking hand.
Tommys hand was shaking because he was nervous to talk to Julius. 
mnli hypothesis: Tommys hand was shaking because he was nervous to talk to Julius.  premise: Tommy heard himself saying idiotically: "The gorse won't be there after all these years." And Julius replied solemnly: "I guess you're right." Tommy suddenly pointed with a shaking hand.