Halliday (2008: 165):
One and the same semiotic system–&–process, that we call “language”, is being manifested (to use the material states by way of analogy) either as liquid or as solid. Spoken language is liquid (some might say it often reaches a gaseous state) and transitory; written language is solid and permanent. There are many such pairs we could use as analogy: mercurial versus crystalline, river vs glacier, dance vs sculpture. What any such pair of terms must suggest is that both states of being are equally rich and equally complex.