Martin (1992: 542):
Taking the nature of socialisation and lexical specialisation into account, a provisional classification of fields can be provided. The first cut is between fields depending on oral transmission and those depending on writing; this opposes home, recreation and trades to public administration, humanities, social science and science (or in more general terms, oral cultures to literate ones).
 The 'oral' vs 'writing' distinction is a distinction of mode (textual), not field (ideational). The confusion is thus in terms of metafunction.
 Many trades, of course, require some form of (literate) tertiary education.