1Ure J. Lexical Density and Register Differentiation [C] // Perren G, Trim J L. Applications of Lin- guistics. London: Cambridge University Press, 1971: 443-452.
2Halliday M A K Spoken and Written Language [M]. Victoria, Australia: DeakinUniversity, 1985.
3Stubbs M. Lexical Density: A Computational Tech- nique and Some Findings [C] //Coultard M. (ed.). Talking about Text: Studies Presented to David Brazil on His Retirement. Birmingham: English Language Research, university of Birmingham, 1986: 17.
4Biber D. University Language: A Corpus-based Study of Spoken and Written Registers [M]. Am- sterdam: John Benjamins, 2006: 38-41.
5Halliday M A K, Hasan R. Language, Context, and Text: Aspects of Language in A Social- semiotic Perspective [M]. Geelong: Deakin Unve- rsityPress, 1985: 48.
6Halliday M A K. An Introduction to Functional Grammar (2nd edition) [M]. London: E. Arnold, 1994: 106.
7Eggins S. An Introduction to Systemic Functional Linguistics (2nd edition) [M]. New York:London: Continuum, 2004: 213-253.
8Laur6n U. Some Lexical Features of Immersion Pupils' Oral and Written Narration[J]. University of Lurid Working Papers in Linguistics, 2002 (50) : 63-78.
9Langer J. Children Reading and Writing: Structures and Strategies [M]. Norwood, NJ: Ablex, 1986.
10Bereiter C, Scardamalia M. The Psychology of Written Composition HVYA. Hillsdale, N J: Lawrence Erlbaum Associates, 1987.