Someone who walks through a beis hapras on stones that he does not cause to move, [or] on a person or an animal that are very strong, is tahor. [However, some-one who walks through a beis hapras] on stones that he causes to move, [or] on a person or an animal that are not strong, is tamei. Someone who walks in the land of the nations on mountains or rocks is tamei; in the sea or in silt is tahor. What is silt? Any place to which the sea rises in its fury.