We are referring to a prepositional phrase as something that expresses a ʻminor processʼ, and interpreting a preposition as a kind of mini-verb [...] Typically, the prepositional phrase functions as what we have called a circumstantial element in the clause. Apart from circumstances of quality, which are mostly expressed by adverbial groups, the usual realization of a circumstance is as a prepositional phrase.
- Halliday (1985), a pag.142