Chemical weathering is the process by which rocks are broken down by chemical reactions. There are different types of chemical weathering. Hydrolysis is the chemical breakdown of a substance when combined with water. The most common example of hydrolysis is feldspar in granite rocks changing to clay.
Racially Africans originated in Africa, Europeans in Europe and so on. (before that the human species originated in Africa, but that's ancient history) African Americans are an ethnic group, or a subrace of the African race.
According to ancestry.com, the average African American is 65 per cent sub-Saharan African, 29 per cent European and two per cent Native American. Many are not ready to hear about their white ancestry, and some DNA testing labs now offer counselling to customers in lieu of their test results being revealed.