A peoples History of the United States According to Zinn, what is the interpretive effect of Samuel Eliot Morison’s mentioning the truth of “genocide” alongside “the telling of a grand romance” about Columbus? [pp. 7-8]

One can say from reading the passage in Zinn's work that the study of history is quite interpretive.  In his work on Morrison, who was a fine historian in his own right, Zinn points out that Columbus's discovery of the New World opened up a period of genocide against an entire hemisphere of people.  Though, soon after he says this, Morrison also points out that this was not intentional on Columbus's part, but rather a byproduct of his will and drive and that it should not cast any shadow on Columbus's achievement.  Zinn also points out that the sanitized version of Columbus is what is taught in grade schools.  Zinn criticizes this as not entirely accurate and he demonstrates that the telling of history has a great deal of ideology in it--a lesson in critical reading that he gives his students early in this textbook.  
http://www.historyisaweapon.com/defcon1/zinncol1.html