Ohalos 18:6

If a person walks through a beis hapras on stones that can’t be moved, or if he rides on a person or on an animal of great strength, he remains ritually clean. If he walks on stones that move, or if he rides on a person or on an animal whose strength is insignificant, he is rendered ritually unclean. One who travels in a land of foreigners over mountains or rocks is rendered unclean but he remains clean if he travels by sea or an a cliff. By a cliff we mean a place to which the sea rises during a storm.

Ohalos 18:7

Let’s say that a person buys a field in Syria close to the border of Israel: If he can enter the field in ritual purity, it is ruled clean and is subject to tithes and sabbatical-year produce; if he he can’t enter it in ritual purity, it is ruled unclean but it is still subject to tithes and sabbatical-year produce. Non-Jewish dwellings are ruled unclean. A non-Jew must have lived there for 40 days to obligate them in an inspection; this is so even if he lived there without a woman. If a (Jewish) servant or a (Jewish) woman watched the dwelling, then no inspection is required.