What is the topic of this talk?

Scientists believed that when the oceans were young, they contained only a trait of salt, and that the level of salinity have been growing gradually. Over the years the rain had worn away the rocky mantle of the continents. Minerals released by this erosion had been carried to the sea by the flow of river water. This water added several million tons of salt to the ocean annually. There are other sources of oceanic salinity too.  One source is under water volcanic action. This eruption occurs along the mid-ocean range and release great amount of salt each year. However these sources of salt don’t increase the actual mineral composition of water by much, because of the enormous chemical requirements of marine life. Oyster, clam, snails and other louse use salt to build shells and skeletons. Coral reefs also contains large amount of salts. Also some salt leave the ocean through the process of evaporation. As salt escape into the atmosphere, they go into the creation of rain drops. This is why the coast received more rain than land area. You see, more salt existence in the atmosphere near the ocean, thereby attracting increased moisture and producing rain.