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.
Again, here are the statistics: Whereas virtually all African Americans have a considerable amount of European ancestry in their genomes, only 19 percent have at least 1 percent Native American ancestry, and only 5 percent of African American people carry more than 2 percent Native American ancestry.