Read it through once

We now come to matters more didactic. Diamonds are not imbedded in rock ledges fifty miles long, like the Johannesburg gold, but are distributed through the rubbish of a filled-up well, so to speak. The well is rich, its walls are sharply defined; outside of the walls are no diamonds. The well is a crater, and a large one. Before it had been meddled with, its surface was even with the level plain, and there was no sign to suggest that it was there. The pasturage covering the surface of the Kimberley crater was sufficient for the support of a cow, and the pasturage underneath was sufficient for the support of a kingdom; but the cow did not know it, and lost her chance.