Read it through once

Such mudbanks soon become pierced by roots in every direction. Then the leaves of the mangroves themselves, as well as silt, soil, and rubbish floating in the water, gradually accumulate about and around these roots. This must raise the level of the ground. Eventually the soil becomes hardened and is above the level of the water. When this happens, the mangrove, which likes salt water about its roots, becomes unhealthy and the ordinary jungle trees kill it and take its place. Thus in course of time, when the jungle is cleared, fertile ricefields may be thriving on what was once a pure, or rather impure, mudbank.