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.
The new scholarship suggests that the West Africans contributed primarily to Euro-American culture whereas people who came from the vast Bantu speaking areas of Africa, to the east and south of West Africa, are those most likely to have left an African cultural heritage to African Americans.