Why is sea water very high in salt content?

The average salinity of sea water is about 3.5%. One cubic mile of sea water contains 166 million tonne of salt. The salts of the sea come from the breaking up of rocks and gradual wearing away of mountains releasing salts that are washed down by rainwater. It is estimated that more than 400 million tonne of dissolved salts are brought to the sea every year by 27,000 cubic km of water. The second source of salt is rocks beneath the ocean bed. There has been a constant slow addition of sea salinity due to evaporation from the ocean surface, combined by discharge from land.