Brigham Young:

... My remarks in the first place will be upon the cause of the introduction of slavery. ... Now, says the grandfather, I will not destroy the seed of Michael and his wife, and Cain, I will not kill you nor suffer anyone else to kill you, but I will put a mark upon you. What is that mark? You will see it on the countenance of every African you ever did see upon the face of the earth or ever will see. Now I tell you what I know, when the mark was put upon Cain, Abel's children were in all probability young; the Lord told Cain that he should not receive the blessings of the Priesthood, nor his seed, until the last of the posterity of Abel had received the Priesthood, until the redemption of the earth. If there never was a prophet or apostle of Jesus Christ [that] spoke it before, I tell you, this people that are commonly called Negroes are the children of old Cain. I know they are; I know that they cannot bear rule in the Priesthood, for the curse on them was to remain upon them until the residue of the posterity of Michael and his wife receive the blessings, the seed of Cain would have received had they not been cursed, and hold the keys of the Priesthood until the times of the restitution shall come, and the curse be wiped off from the earth and from Michael's seed. Then Cain's seed will be had in remembrance and the time come when the curse should be wiped off. Now, then, in the Kingdom of God on the earth, a man who has the African blood in him cannot hold one jot nor tittle of Priesthood. Why? Because they are the true eternal principles the Lord Almighty has ordained, and who can help it? Men cannot, the angels cannot, and all the powers of earth and hell cannot take it off...
But let me tell you further, let my seed mingle with the seed of Cain, that brings the curse upon me and upon my generations; we will reap the same rewards with Cain. In the Priesthood, I will tell you what it will do. Were the children of God to mingle their seed with the seed of Cain it would not only bring the curse of being deprived of the power of the priesthood upon them but they entail it upon their children after them, and they cannot get rid of it. If a man in an unguarded moment should commit such a transgression, if he would walk up and say cut off my head, and kill man, woman and child it would do a great deal towards atoning for the sin. Would this be to curse them? No, it would be a blessing to them; ...
It is the greatest blessing that could come to some men to shed their blood on the ground, and let it come up before the Lord as an atonement. You nor I cannot take any more life than we can give. ...
We know there is a portion of inhabitants of the earth who dwell in Asia that are Negroes and said to be Jews. The blood of Judah has not only mingled almost with all nations, but also with the blood of Cain, and they have mingled their seeds together. These Negro Jews may keep up all the outer ordinances of the Jewish religion, they may have their sacrifices, and they may perform all the religious ceremonies any people on earth could perform, but let me tell you, that the day they consented to mingle their seed with Canaan, the Priesthood was taken away from Judah, and that portion of Judah's seed will never get any rule or blessings of the Priesthood until Cain gets it. Let this church which is called the Kingdom of God on the earth, we will summons the First Presidency, the Twelve, the High Counsel, the Bishopric, and all the Elders of Israel, suppose we summons them to appear here, and here declare that it is right to mingle our seed with the Black race of Cain, that they shall come in with us and be partakers with us of all the blessings God has given to us. On that very day, and hour we should do so, the priesthood is taken from this church and kingdom and God leaves us to our fate. The moment we consent to mingle with the seed of Cain, the Church must go to destruction; ... (1)

Source: The Teachings of President Brigham Young, Vol. 3, 1852-1854. Fred Collier, ed. Collier's Publishing, 1987. 41-48, in The Complete Discourses of Brigham Young, Ed. Richard S. Van Wagoner, Smith-Pettit Foundation, Salt Lake City (2009), http://bit.ly/BY-discourses