dash or comma before summative modifier?

[Yonsu99]

Hello, I have a question about writing style. When we use summative modifier, which should I put before it between dash(--) and comma(,)? The following sentences I observed have either dash or comma, so I'm a bit confused. Are there any rules how to use dash and comma as in the underlined phrases?

"The total cost to the country of all the various acts of incompetence and malfeasance in the Harding administration has been put at $2 billion--a sum that goes some way beyond stupendous, particularly bearing in mind that Harding's presidency lasted just twenty-nine months."


"[One] method of connecting the trailing element to the main clause is with a word that restates or sums up what has been said, a technique I am using in the sentence you are reading now."
(Stephen Wilbers, Keys to Great Writing. Writer's Digest Books, 2000)


"For some time it has been a noisy pageant--laughter, gunfire, war whoops, the intoning of sermons, a politician's blast, the cries of love and pain, iron-shod wheels on cobblestones--all in all a terrible racket."
(Quoted in The Oregon Blue Book, 1997)




"The headstone stood above seventeen layers of unrecorded East Londoners: cats, rabbits, pigeons, pebbles and rings, all impacted in the heavy clay."
(Iain Sinclair, Lights Out for the Territory. Granta Books, 1997)

 

[bhaisahab]

I would say that the dashes in your examples are replacing semi-colons.

 

[Barb_D]

I have never heard the phrase "summative modifier" before, but thanks to your helpful link, looked it up. The first does not modify your entire clause; it's an appositive to "$2 billion." The second does not modify the entire phrase; it is an appositive to "noisy pageant."

However, both an em-dash and commas are used.

 

[MikeNewYork]

In my opinion, semicolons would be incorrect in those uses.

 

[bhaisahab]

In my opinion, semicolons would be incorrect in those uses.

I disagree.

 

[Raymott]

Before deciding whether a semi-colon was appropriate, I'd ask myself whether the second phrase/clause was a full sentence. Since in none of the examples does that occur, and since there is no other apparent indication for a semicolon, I would not use one.