coherence and cohesion

Dear all,
First of all, I would like to tell that the text given is very long and and is full of examples . Reading such a text is time-consuming and needs special attention to grab what the writer wanted to tell readers more or less. Before reading the text, I understand that the two words- cohesion and coherence- are quite close in term of meanings but after reading it my thought has changed.
As long as I understand, Williams, the writer, described cohesion as the connection or the relation between the story idea and the supportive sentence to form the whole story.
The writer said good cohesion must rely on clarity and context, management of the flow of information, beginning of good sentence, and finding topic of sentence. So good writers are necessary to apply these rules to their writing which would be easier to read.
Coherence on point of the writer's view means the connection and the relation between ideas, arguments and statements in the story. The writer yet told us that coherence is abstract: we cannot point to it as we can point to a noun( p.81). That's why he was giving us many examples to learn so that we will know whether each writing is incoherent or not.
Thanks /anucha