5.2.1 Cohesion
There are two main types of cohesion：grammatical cohesion and lexical cohesion.Halliday and Hasan（1976）further elaborated cohesion into five categories：reference，ellipsis，substitution，conjunction and lexical cohesion.The grammatical cohesion devices are mostly the function words devoid of lexicalmeaning.Functioning in two ways：reiteration and collocation，the lexical cohesion devices are mostly contentwordswhich have clear notions.
High frequency words illustrated in Section 4.1.2，such as determiner（the，a），prepositions（of，in，to，for，by，on，at，with，from），conjunctions（and，as），copulas（is，was，are），pronouns（it，that，you）play a significant role in cohesion.He has been demonstrated in Section 4.1.3.2 as an example for reference，and expounded in Section 4.1.3.3 for conjunction.Both reference and conjunction help to achieve gramm atical cohesion.On the other hand，the following example will lead us to lexical cohesion.
Exam ple 20：
（1）Out of all of America's symbols［variation］，none has proved more enduring or evocative than the Statue of Liberty［spot name］.This giant figure［variation］，torch in hand and clutching a stone tablet，has for a century acted as a figurehead for the American Dream...
The statue［variation］，which depicts Liberty throwing off her shackles and holding a beacon to light the world.（OETT）(https://www.daowen.com)
（2）The Great Wall of China［spot name］was built mainly to protect the Chinese Em pire from the Mongolians and other invaders.It［reference］was first built in the 7 th century B.C.when China was still divided into many small states.The construction of the Great Wall［reiteration］had never ceased for nearly all the Chinese feudal dynasties.
The Great Wall of China［reiteration］extends 4，000 miles westward：From the China Sea town of Shanhaiguan to Gansu Province.（TETT）
Passage（1）applies“symbols，figure，statue”to stand for“Statue of Liberty”.The lexical cohesion is achieved through equivalence which is one of the principles that underlies variation.It reflects the event-line and participant-line of a text on the semantic stratum，and reveals the theme of the text.
Passage（2）uses it or reiteration to denote the Great Wall.As English always avoids lexical repetition，“elegant variations”（Leech＆Short1981：247）in OETT seemsmore idiomatic.In this respect，TETT is lack of lexical variation and seem monotonous.