What subject would you say geology comes under? Is it a branch of chemistry, physics or geography. You could even say is was part biology. Talking of geography, do you find it strange that there is a subject that comprises both humanties and hard science? OTOH you have the formation of ox bow lakes and scree; OTOH you have the Norwegian leather industry and the Swiss canton system. If you separated physical geography out, where would you place it? It wouldn't fit easily under either chemistry, biology or physics. It would be either a separate science or an amalgamation of the lot. Likewise, would you say metallurgy is a branch of chemistry or physics?