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.
African American topics. African-American history is the branch of American history that specifically discusses the African-American or Black American ethnic groups in the United States. Most African Americans are the descendants of Africans forcibly brought to and held captive in the United States from 1555 to 1865.