I have in my notes that in  an 1899 essay, MT wrote,

  ³It would not be possible for a humane and intelligent person to invent a
rational excuse for slavery; yet you will remember that in the early days of
the emancipation agitation in the North the agitators got but small help or
countenance from any one. Argue and plead and pray as they might, they could
not break the universal stillness that reigned, from pulpit and press all
the way down to the bottom of society--the clammy stillness created and
maintained by the lie of silent assertion--the silent assertion that there
wasn't anything going on in which humane and intelligent people were
interested.²

Anyone know what the source is for this quote?

Thanks,

Harold K. Bush, Jr.
Saint Louis University