"We pray the God of mercy to deliver us from our present Calamity," wrote Patrick Breen on the first day of 1847 as he and others in the Donner party awaited rescue from the snowbound Sierras. His famous diary appears in Overland in 1846, edited and annotated by Dale L. Morgan. This handsome two-volume work includes not only primary sources of the Donner tragedy but also the letters and journals of other emigrants on the trail that year. Their voices combine to create a sweeping narrative of the westward movement.   Volume I concentrates on the experiences of particular pioneers making the passage--their letters and diaries describe omnipresent dangers and momentary joys, landmarks, Indians encountered, disputes within the companies, births and deaths. Volume II, also based on contemporary records, offers a broader but no less vivid view of what it was like to go west in 1846 and pictures what was found in California and Oregon.