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.
Modern Human beings evolved in Africa from a variety of hominid species over a few million years. The first humans to leave the African continent were Homo erectus (upright man), who migrated northeast about 2 million years ago when present day deserts were grassland.o when modern humans decided to look west rather than east while pondering their future in a cold Central Asia they would have found Europe already occupied by Homo neanderthalensis, who had separated from the main evolutionary line some 500,000 years ago.