What is Non-Garrisonian?

Garrison was a person who was an abolitionist.

Abolitionist = person who wished to end slavery and slave-trade 廢奴主義者

Non-Garrisonian abolitionists = people who did not want to end slavery

disavow = deny/reject 否認/抗拒

miscegenation = interracial marriage, especially in the US back in the days, between a black (slave) and a white person 異族通婚

People who did not want to end slavery denied/rejected marriage between members of different races and the motive to join the revolution in abolishing slavery or slave-trade.