What is the mark that God placed upon Kain?

Debugonnawinn: Dark skin? As in being genetically adapted to living in beautiful, warm, sunny climates as opposed to white skin which is ideal for cold, dark, miserable climates? If anything, white skin sounds like a curse and dark skin a blessing. Surely the garden of Eden had the former climate. I'm not racist but I'm tired of these white supremacist Southern Baptists trying to find Biblical justification for their racist tendencies.