The Colon.
1. The colon is usually a mark of specification. Thus, “The old idea of education was simple: reading, writing, arithmetic.” A fine distinction of logic can be shown by using it: a general statement may be followed by a colon, after which the details that explain the statement may be given. In the following sentence the colon specifies what is meant by fine character. “He was a fellow of fine character: brave, honorable, free from false pretense.” Usually the colon separates clauses that are logically, if not grammatically, in apposition with each other.
2. The colon introduces a formal or long, the comma an informal or short, quotation. “He answered, ‘I will work while the day lasts.’” “The Declaration of Independence begins as follows: ‘When, in the course of human events.’”
The Dash.
1. The dash shows a sudden break in the thought. Thus: “We were hurrying onward—but first let me tell what happened before that.”
2. The dash sometimes precedes a summing up. Here it usually follows a comma, since the members of the series are set off by commas: “Chaucer, Shakespeare, Wordsworth,—very many of our great poets indeed, were at home in the country.” Sometimes the dash is used when there is no real summing up, but an appositive phrase is added, as a further explanation. For an example, see the last sentence of the next paragraph,—and this sentence also.
3. The dash, like the comma, is often used to set off a parenthetical expression. (See 2, under the comma.) Examples: “His father—that iron gentleman—had long ago dethroned himself.” “He was a man—the reader must already have perceived—of easy, not to say familiar, manners.” Note that in these examples no commas are used with the dashes, because if the parenthetical words were lifted out, the sentence would close up without punctuation. But suppose the sentence were such that it could not close up without punctuation; then the comma would be needed. The comma in “His father being angry, he felt afraid,” remains when the parenthesis is inserted: “His father being angry,—that iron gentleman,—he felt afraid.” Note that in such a case a second comma is used,—with the second dash.
Written Exercise.—Copy and punctuate the following sentences from Stevenson. In the first is there not a choice of punctuation after “difficulties”?
1. All the way down we had our fill of difficulties sometimes it was a wear which could be shot sometimes one so shallow and full of stakes that we must withdraw the boats from the water and carry them round