the narrow shining line upon the earth resolved itself swiftly into a river of great immensity : slow and very broad , fractured into many smaller streams , all winding together past rocks and small grassy islets towards a narrow crack in the earth like an eggshell broken down the middle , where the river suddenly boiled up and plunged into the thunder of a waterfall more vast than anything laurence might have conceived , the gorges of its descent so full of white-plumed spray that its base could not be seen .