Read it through once

First a green herb rooted itself every here and there amidst the desolate boulder-clay or perhaps in a crevice where good earth had accumulated. Then the scattered colonists began to form groups; soon patches of green moss united them. Then a continuous green carpet could be traced over a few yards here and perhaps on a few feet somewhere else. But when things had got as far as this, progress became much more rapid, and soon the whole site of the future cornfield was covered over by a continuous green carpet. Only, every here and there, hard stones and uncompromising trap-rocks remained still protruding from the green covering.