I would say that in very fast moving water such as in a river, the cobble size could be transported because cobbles are 64-256mm so are quite big and in a quiescent stream won't move much but in a freshet say in a steep mountain stream they can be rolled along such as I observed in the local streams at a mining town during the Hurricane Fifi in Honduras in 1974.
introduction of deadly diseases, forced slavery by some nations, and forced colonization by Europeans
Third planet is earth.. which has it's atmosphere and support life.,.
that's very different from all other planets<span />
